Legally Prevent Adult Stepson From Moving Back

My wife and I have been married 2.5 years. Her adult son has been living for 6 months in an apartment while in school. We are in Michigan.

The first 2 years of our marriage was hell due to him living there. The last 6 months has been our honeymoon and really fantastic and stress-free!

He now wants to break his lease and move back in with us. My wife wants to allow it because "it will be better this time".

How do I legally prevent this from happening?

Edit: our marriage wouldn't have lasted had he not moved out and will quickly disintegrate if he moves back. I sold my home and moved into hers when we married.