I have to say for not knowing what should have been told to you....you did an excellent job of reacting! You kicked him out. Either he cleans up or not. It sounds promising.
Keep you boundaries of 3-4 months and evaluate the next step. No hurry but have a plan A and plan B.
Drug addiction is an outright WAR! The addict has the responsibility of maintaining sobriety and recovery. The family has their own life to live.
We can't make them want it for anyone but themselves.
One thing to consider is he was keeping "a secret" and probably had no support during his period of sobriety. If he ever returns to you...he needs to maintain a recovery program (support).
Good luck....this is a lifetime deal if you choose to stay.