You can't help him. That's the point. Only he can help himself, and it sounds like he isn't anywhere near the point of even realizing he has a problem, much less addressing it.

An angry, alcoholic 19-year-old with three DUIs, a suspended driver's license, and a jail term isn't someone you're going to be able to help. Until he helps himself, he'll keep putting his life and that of others at risk. You can stick with him and enable him, let him keep yo-yoing your emotions, and be there to financially support him and bail him outta jail and other legal problems. But you can't help him. You can only help yourself, and the best way you could do that is by finding someone healthier unless you want to be pulled under with him.