I've had some experience with a relative who was an addict.
Yes you can be of help, somewhat. If you care about him, always let him know that you care about him, as a person.
You can remind him, "Don't let your problems define who you are. Don't call yourself a "drug addict," instead say, "I am a person who has a problem with drug addiction, but I am still a person." That helps decouple the problem from the person. It's the difference between being the problem, and having a problem. If you are the problem, then nothing can be done about it, other than wish you were someone else. On the other hand, if you have a problem, then the problem can be addressed.
[I then go on to use the shorter term "addict", but whenever I say "addict" I really mean, "person who has a problem with addiction."]
If he's an active addict, then likely anything of value you have may mysteriously "disappear". Keep that in mind. No matter how well you hide it, they'll find it. Lock valuable stuff away, preferably somewhere else he can never get to.
You can also set your boundaries. If he's high and that's affecting you, focus on what you need. "I need peace and quiet. You're high state is affecting my peace and quiet. I still care about you, but we need to solve my problem so I can have peace and quiet." That way the problem to be solved is your problem, rather than focusing on his problem.
I highly recommend finding a local Nar-Anon group or Families Anonymous group to go to. There you can meet other people who will understand. The seasoned people may have wisdom to share. It's a good place just to vent about what crazy thing your beau did this week. They may have a phone list of members you can take home with you, so if you find yourself in a crisis you can call one of them, just to hear a familiar voice, who may be able to help calm you down, maybe offer suggestions. I've been going to both Nar-Anon and Families Anonymous for a couple years now.
You can mention that drug addiction is nobody's fault. Imagine a mechanical clock, and inside are all sorts of gears and parts. One day the clock doesn't work right. We say the clock is "broken". We want to fix the clock. We open the clock with the expectation that we will find the broken part. Then we can repair or replace the broken part, and the clock should work again. Now imagine we do open the clock, and we inspect every single part inside, every gear, spring, sprocket, whatever, and we discover that every single part is in perfect pristine condition. And yet the clock failed to work properly. How is this possible? It's possible because there's another possibility that was overlooked. It's possible the overall design of the clock, how all the gears and parts are designed to interact with one another, it's possible the design itself is faulty. A system with no broken parts can still fail, and the failure may be in the design of the system-as-a-whole.
It's the same with systems of people. No one approaches life with the thought, "Of all the careers I could chose, I think "Drug Addict" sounds the most appealing." It just doesn't happen that way. No one plans to become a drug addict. No one choses that. It just happens. And it's probably because of the environment they lived in, which is mostly beyond anyone's control.
People often turn to drugs to escape stress. Initially it actually works, a little, but of course it's a trap, and soon the drugs themselves become a part of the problem.
People who have a problem with addiction can't really use "Self Control" to just stop using. "Resistance is a finite resource in the brain."
Reducing stress is one key to recovery.
Here's a very informative video. It's an hour and 12 minutes, but very informative:
Is Addiction Really a Disease? Dr. Kevin McCauley
You can help by "planting the seed of the idea of getting clean and off drugs" into his mind. Just mention it as an idea, every now and then. Each time you mention the idea, it reinforces the idea in his brain. The hope is to eventually grow that idea to be strong enough that they actually want to take action to stop using.
(BTW if you are an ex-addict, then you have to get out of there or you'll just relapse, because if the drugs are there, your prefrontal cortex is the part that tells you not to use drugs, but that part tires easily, and eventually your midbrain will win out every time and relapse for yourself is inevitable. On the other hand if you've never done drugs, and don't plan to start, then ask him politely to never try to get you to do drugs. If he respects that it's a good sign he may be able to save himself eventually.)
Probably the hardest thing to accept is you can't "fix" him. You can't force him to go clean. The best you can do is lay varous options before him and see if he takes any of them. It has to be his choice, or it simply won't work.
Nar-Anon and FA work by ensuring your needs are taken care of, so you are well mentally and spiritually. We nurture ourselves so that we may then nurture others. When we are well with ourselves, and not stressed out, and not trying to control everything, it puts us in a mood where we can lovingly interact with the addict, and accept that they have a problem, rather than fight it.
Be aware of 3 categories we put people in. The category defines how we treat them.
1. Conformity: The person should not use drugs. Using drugs is a "choice". The person made the wrong "choice".
2. Medical: Addiction is a medical disorder.
3. Typology: The person is the way he is. Who are we to say he should be different?
If the person is in the first category they are a "criminal" and criminals are sent to jail and prison.
If the person is in the second category, they are sent to a doctor to get the help they need.
If the person is in the third category, they end up in jail and prison because we can't stand them.
Addiction is a medical disorder of the brain. And most addicts secretly or openly want to get clean. They just don't know how. There are very strong forces going on in the midbrain. Addicts need a lot of love and encouragement to get clean. At the same time don't let them walk all over you, steal your stuff, use you. Focus on their accomplishments. ("You went 2 weeks clean before relapsing. That's a big accomplishment! Congratulations! Keep trying!")
Ultimately they may get arrested, and that may be a good thing, because it's harder to use when you're locked up in jail. (A lot of drug users detox in jail, and it ain't pretty. They don't do much to help you along. You just detox the hard way. Not fun but it works.) Keeping them clean when they get out is all about teaching them skills to reduce their stress. When stress is low, their prefrontal cortex is in control and they don't relapse. If stress goes high, or if they get angry, the prefrontal cortex shuts down, they literally can't think and reason because that part of the brain went offline, and they end up doing things they would normally not want to do.
The other thing is to watch out for an overdose. If the person passes out and cannot be awakened, no matter how much you shake them, then that's an overdose. Make sure they don't end up face down in a pillow or they will suffocate and die.
I think you can also purchase stuff from the pharmacy which counteracts an opioid overdose. Might be good to get some of that and have it on hand. Let me look,... looks like it's called?naloxone. May or may not require a prescription. Ask. It may not. [Edit: may not work for cocaine. I'm not sure. You'll have to check.]
Support groups help reduce stress by making us feel connected with other people.
Mindfulness Meditation helps reduce stress by activating the Parasympathetic Nervous System. (Same with Therapeutic Yoga, Qu-Gong, Tai-Chi, and other mindfulness motion exercises.)
A desire to not use drugs can grow and eventually overpower the desire to use drugs. These two desires reside in two different parts of the brain, and they fight it out like two wolves. Which wolf will win? The one you feed.
OK that's what I can think of. Mostly what I've learned from dealing with my relative, who actually did succeed getting clean. He does relapse on rare occasions, and he's still learning from those relapses. The key is to reduce stress, both yours and his. And break old habits by creating new habits that replace them.
Best wishes and good luck!