Thank you for writing. I have been in a similar position. There is an entire organization for people who know someone with a drug or alcohol addiction problem, and in his case it goes along with depression, and there's a whole wealth of knowledge and experience that I can't squeeze in here but I'll try.
Basically,one big primary goal is, to always take care of yourself first. In fact, when you learn how to take care of yourself when you are stressed, then you by example show him how he can take care of himself when he is stressed, or depressed. (Ways which don't involve drinking.)
One primary way is to go to a support group, where one finds other people who understand, and care, and no matter what, they listen to your story, and they still accept you afterwards, and THAT can be very healing and reassuring.
There may be a Al-Ateen or Nar-Ateen group in your area, or something similar. These are groups for teens who know someone who has an addiction problem. About half of everyone who has a mental illness problem, such as depression, or bipolar (moodswings from mania to depression), or other mental disorders, about half of them also develop a drug or alcohol addiction problem, so it's very common to find both. People drink and use drugs to escape. To escape the crappy world they live in. To find happiness when they are not happy. To self-medicate their mental illness problem.
Here's a quick lesson on brain addiction and how it happens. (I've been studying this for a few years now.) Understanding this helps lead to understanding how to pursue recovery. Pretend your arm is your spine. Make a fist, that's your midbrain. Place your other hand over your fist, that's your cortex (the wrinkly stuff when you look at the outside of a brain.) (The brain has more parts, but for this discussion we're interested in the "midbrain" and the "cortex" (or "frontal cortex" or "prefrontal cortex". I'm not an expert yet.)
The midbrain is the most primitive part. This is where emotions come from. The midbrain's purpose is to keep you alive and pass your genes on to the next generation. If you see a scary tiger running towards you, your midbrain goes, "Yikes! Run!" and you run. If you see delicious food, your midbrain urges you to approach the food, reach out, and eat it. If you see a handsome man, your midbrain urges you to approach and engage.
The midbrain tells us what to do, without bothering to explain _why_ we should do these things. It doesn't explain why we should run from scary tigers, why we should eat food, or why men are attractive to us. It just tells us what to do, and a very long time ago that was good enough.
Later we developed a prefrontal cortex. This is where rational thought resides. This is where we imagine possible outcomes, consider consequences, and make rational decisions. The prefrontal cortex can override the desires of the emotional midbrain that just wants what feels good. The prefrontal cortex is however rather slow at figuring things out. If the prefrontal cortex sees a scary tiger running towards you, it might think, "Hmm, that looks like a large tiger running towards me. Let me project into the future possible outcomes. The tiger might jump on me, knock me down, rip me apart with its claws, bite me with its teeth. OK I don't think that would be a good thing to have happen to me. Let's see, what options do I have to deal with this. What can I do to avoid this unfortunate projected outcome? I could fight the tiger with this stick I have. Or I could run away. Hmm, which I should chose? Fight or run? How about we run. Yes, let's try that. Let's run away."
Notice how the prefrontal cortex took a long time to figure out what to do about the tiger running towards you. All the while the midbrain already has an action plan. The midbrain, upon first noticing the tiger, emotionally thought, "Yikes! Run!" However, the prefrontal cortex is overriding the midbrain. The prefrontal cortex is saying, "Hold on there midbrain, let's not be hasty about this. Let me think about this." The midbrain however, sees the prefrontal cortex as a _liability_ in this scenario (This cortex is going to get us killed!), so the midbrain literally shuts down the prefrontal cortex--it squeezes off the blood supply to the prefrontal cortex, literally shutting it down, allowing the midbrain to gain control so it can quickly make you run away. That's how evolution resolved the problem of tigers and the slow thinking cortex. The solution works great for tigers. It's an absolute disaster when it comes to addiction.
That's the tricky part about addiction and the brain The rational thinking prefrontal cortex is usually in control; however, under duress the midbrain can shut down the prefrontal cortex, so you literally can not think rationally anymore, and you just react emotionally. The prefrontal cortex is the part of the brain that decides, "I want to stop using drugs / stop drinking alcohol." As long as the person remains in a calm serene happy state, the prefrontal cortex remains in control. However, if the person becomes stressed (and let's face it, our modern day society creates stress everywhere), the midbrain can gain control by seeing the prefrontal cortex as a liabiliy and shutting it down so the midbrain can get what it wants. The midbrain, when stressed, sees this as a potential life-or-death situation, and to save your life,it will literally shut down the rational thinking prefrontal cortex, and take over control, and say, "What will get me out of this stress? I know, alcohol will fix this. Get me that alcohol." And the person goes off and drinks alcohol.
One more thing to note, if you look at a diagram of the brain, notice the midbrain is located BETWEEN the cortex and the spine. All decisions the cortex makes must go through the midbrain before they reach the spine and the rest of the body. The midbrain must approve anything the cortex decides to do. The midbrain, because of where it's positioned, can also take over control of the body, and leave the cortex out of the loop. The part of the brain that decides "I don't want to drink anymore" isn't the part of the brain that becomes addicted. — it's the primitive midbrain that becomes addicted.
If people could simply decide to stop using drugs and stop drinking alcohol, then there wouldn't be any alcoholics and addicted people in the world. People would simply chose to stop drinking and using drugs. But it's not that simple. Addicted people DO chose to stop drinking and to stop using drugs — and then they keep drinking and using drugs. Which is VERY confusing to them, and everyone else around them!
It's like a rider on a horse. Who really is in charge? Usually the rider is in control of the horse. But if the horse gets spooked, the horse can run off, with the poor rider being taken along for a ride he's not in control of. That is the life of the alcoholic an drug user. A person uses drugs, it becomes a problem, they decide they should stop — and that's when they discover they literally can not stop. It's too late. They are already addicted.
This is the dilemma of someone who has developed an addiction problem. They rationally decide they no longer want to do drugs / drink alcohol; then they get stressed, and they find themselves doing what they decided they didn't want to do anymore. They literally can not control themselves anymore, because the midbrain is shutting down their prefrontal cortex where rational thought occurs.
The solution then, is to learn how to mitigate stress, learn how to relax, learn how to stay calm, learn how to find happiness without using drugs/alcohol. When in a calm serene happy state, the prefrontal cortex remains online and active, you remain in control.