This is not actually a legal contract. A contract requires consideration. Both parties have to lose something in order to gain something. Here, he is losing the drug use but is gaining the ability to stay in the house. BUT you are gaining nothing, and losing nothing. Hence, not a contract.
This is not enforceable. And of course he knows that-if one day down the road he decides to use again, he knows that there is not a thing you can do about it. And you best believe, if that happens, he will try that excuse.
However, that doesn't mean you can't do it anyway. IF he relapses, you can say he knew of the consequences ahead of time and this is no surprise. And then of course, you also have it all written out on paper to remember what you said-because you may forget. And then you can act on it.