Well since he is in jail you can always write him. Let him know that you will come when you can but don't set a specific day or anything unless thats what you want.
As far as him getting help I think pple who go FIND help get better help than pple who are "caught" and have to get it. (only my opinion)
You seem like a very sweet giving person and it is hard to walk away when someone we care about needs help, but this is a great oppurtunity for him to learn to help himself. Like I said write him, send him some self help books. Other than that you can't fix him. Let him start to depend on himself. When pple are in jail they usually do alot of soul searching and usually have good intentions while they are there but then they get out fall back into the same cycle with the same pple and its a vicious cycle. So if you want to help let him start helping him self. (this is just my opinion)