Hi there, Sorry to interrupt - I'm new here! But, i just want you to know that my two brothers, 2 yrs either side of my age were like this. They both sorted themselves out in their mid to late 20's. They didn't do the drugs, but they were in and out of prison for alsorts of things for years and I always thought they had no morals. They'd steal cars and motobikes, break into superstores, get involved in fights, mix with the wrong types, etc. I remember lecturing them quite a bit and they'd tell me what they knew I wanted to hear, then do the same things again. I always kept in contact with them though and they remember me doing that. In fact no-one gave up on them. We just kept out of their lives as much as possible.
It sounds like you're way too involved in your son's life, and I guess that's because he lives with you. You have to accept he simply doesn't have the same moral code and compassion for others that you have. Accept that your son's the way he is now, and that you're worlds apart in these respects. I don't think any amount of talking to him will get him to be the person you want him to be deep down right now. But, it's good to do it now and then, as one day it will hopefully sink in. He has to want to change his ways, and to feel it inside. It could take time. As his mother, who loves him, I'd be there for him to come to for support and love, but let your duties as mother to a now 19yr old end there. I'd find a way so you're not so absorbed in his world and so you can focus on the younger ones. If it means your son stays with you less or moves out completely that might be what's required. But you being so worried, so involved, so upset, will not make him stop doing what he's doing, not now. He's living in a different 'world' and is surrounded by very different people to you. I really think you need to let him go and live his life. It might be shameful and embarrassing to you, but there's little you can do about it right now. What you can do though, is make life at home happier for you, your partner and the other kids. Fix what you can fix, improve some things that you have control of, and just give support to your older son when it's asked for. I'd make it clear to your 19 year old son what you see your role as (emotional/moral support, love, etc.), make sure you have a response so you know he's taken it in and then relinquish yourself of the rest of what you do - like someone who has to give up smoking -use pure will power to stick to it and give up the stuff that's bad for you and the rest of the family. You might fail once or twice along the way, but remind yourself of what your role in your son's life is. It's not to make him be you!
Sorry, if this is not very helpful and certainly wont fix your son now. But, I just wanted you to know that a lot of kids living this life do fix themselves in the end, especially when they have a family who've not given up on them and when they gain responsibilities for themselves -just let them fly the nest and live their life as they feel they need to, let them make mistakes and learn from them. And I know his actions affect others, but, I don't think it's all down to your son and you can't protect all these people. They each need to take responsibility for their own decisions and not be influenced. They will learn from this hopefully, and some do learn the hard way! But at least they're learning these huge lessons while they're young. Also, your son will be vulnerable to dodgy characters no matter where he lives (at present). It's not something you can protect him from - not at all, but you can protect the rest of the family from it. Make better what you have control of.
Best wishes with what you decide to do.