You are right-- how he will do will depend on a number of
factorsFactor ix complex, including his desire to change, his willingness to learn from others, his ability to remember the bad times, the quality of the counselors.. staying clean usually requires adopting
recoveryRecovery position - series as a 'lifestyle' to some extent. He will have to avoid other intoxicants too-- not just meth. And he will have to tolerate a good hard look at himself, and a willingness to see what is really going on.
The biggest favor you can do him is to do anything in your power to keep him there. If he wants to leave, tell him that if he does you will never help him financially-- ever. Make sure he stays there, and if there is any chance of getting him into a halfway house, do that too. The longer he is in a recovering community, the higher his chances for staying clean.