Hi,
Change IS possible... The thing is some people never change...
It is up to him to put the effort in and get clean. Remember that - up to him.
You done a brave and couragous thing by leaving him before. And if it comes to that again remember you have the courage and power to do it again.
I dont really have any advice for you, but dont let yourself get sucked into his addictions, by that meaning caring and looking after his every need.
It is his time to chose what he wants. It will be difficult for him to get clean (if he is still using), but it is difficult for you NOW.
Right now you look after no.1.........you...
Hope things work out for you, for the better