Hi ejohn, have you set some boundaries for yourself as to what you will allow in your home? For instance, what happens if your son continues to use? Is he expected to contribute to the household, or pay for his board in kind? Get a job? Attend re-gab, counselling? I could be useful to get clear in your own mind.
I think what the other posters are getting at is that some addicts will continue to use, and that having a home and board just makes it easier for them to keep on as usual because they aren't experiencing the consequences of their actions.
I totally see your POV, that getting him away from the sick relationship will help him.