Three months isn't long at all to be so invested in what happens to him. If he has been an addict for 15 years you can bet his family knows his issues, have seen this before.
Don't contact them. It isn't your role. Step back and let him live his life no matter how horrible you know his situation is. He is an adult. He will get help when he is ready.
I know when you have concerns about people and a bunch of strangers are saying don't have them, it is because what you describe is classic addict behaviors that others have seen and dealt with. If he hasn't contacted you, then be relieved actually. That means he at least respects you enough to tend to his mess on his own and not suck you into it. Because it will suck you in and can really cause damage (as his stealing from you indicates).
He will get help when he is ready.