What are your boundaries prizefighter? For you - for what you are willing to accept in your life. Not for him. You can't force him to change. You can only change yourself.
Is it ok with you that he drop out of rehab and come home to live with you? If it's not, then why are you letting yourself be pressured into it? Because you don't want to be considered an "*******"? That doesn't sound like a very healthy relationship. It sounds a little onesided.
Say what you mean. Mean what you say. But don't say it mean.
If we don't stand up for what we believe in, for what is important to us, then we are living a lie. A lie that can destroy relationships and lives.
Have you looked into any support for you? Like alanon or codependents anonymous. Or maybe read some books on how to set healthy boundaries in a relationship. Or another good book is called "Codependent No-More" By Melanie Beatty.
If you don't stand. You will fall. And you won't have saved anyone. Especially not yourself.