My opinion on this will likely be unpopular, but I'd do the following, in order.
First, try opening him up. This is different than you opening up to him, and very different than confronting him. I can't think of any situation like this that has gone anywhere with confrontation. If anything, you want to give the middle-point of indifference and direct attack. There's a sweet center to the balance here, and it's important to understand the other person. It's one of those put-yourself-in-their-shoes situations.
If you open up to him with the love that's clearly in your heart, but be
completely honest in doing so, it could work wonders. You need to let him know that not only is he hurting himself, but he's hurting others as well. Be clear that this hurt is both emotional and physical (I'm keeping in mind the stolen property here.) and let him know you're sincerely worried about his well-being. The best part is, if it doesn't work, damage likely hasn't been done. He'll probably blow you off or get offended, but it'll be easy to move on from and go to the next tactic.
Second, you can circumvent him and try helping through gathering support from family and friends. Sometimes we need backup in these situations. See if you can determine who would be on your side with this, especially being open about this with your parents, and see if a good old fashioned intervention is in order. A non-confrontational one!
Third, and I don't recommend this myself but have a cousin who this worked for with his sister, you could try the old 'liquor down the drain' method. Just take his booze and drugs and dispose of them continuously until he catches on that someone doesn't like what he's doing and he runs out of money to keep resupplying himself, at which point he may open up to stopping his behavior. Again, I really don't like this kind of thing. It's way too passive-aggressive and given the stolen property you found in his bag, this method may just lead to him stealing again and possibly hurting someone. I wouldn't want that on my conscience.
Last resort? Turn him in. This is surely tough love, but he's breaking multiple laws in all this. Contact the police about the stolen property, anonymously, and let karma get the better of him. He won't serve nasty time, but he'll get a nice state-funded wake up call. If nothing else works, that should.
Please don't take any of this as anything even close to professional advice. Before anything I'm babbling about, I'd speak to a professional counselor about it. There are resources out there. Read about them first.
You have to keep love ahead of anything here. It's the most important fuel for what needs to be done. Programs that are available to him are his best shot before tough love. Don't forget that he's sick. It's not the flu, but it's an addiction issue. If you don't understand that kind of issue, look into it and gain his perspective so you know who you're really talking to.
Check
this out too.