I did the same with my gf. Put her through some stuff that I’m not proud of. It just turns you into a different person.
So when did this using start? And how old was he?
He probably feels ashamed if he tells the family which I can understand, but it’s common now, really common in afraid to say.
I don’t think it’s a bad thing if you want to stay and help him. It shows you want to make it work, but he will have to put the effort in.
Maybe make him aware its not a bad thing to admit you have a problem. It’s a good thing actually. Because then we can fix it and get our life’s back. Some people are just stuck in a rut and can’t get help or want help, and don’t believe certain things will work. They just need a good push in the right direction. Has he no brothers or sisters that know or you could talk to? Don’t if it would cause problems though.