Just a clarification, he's not subleasing if he is still living there. He has a roommate in the current arrangement. I'm on the same page with you though like I said earlier. He could be in the process of helping someone in need of help. The problem for you is if you let that person in you can't get them out.
As for the lease, you can't just cancel it. He is trying to make a substantive change and you can say that you don't approve of the change and he can accept the lease or walk away with whatever penalties you specify in the lease (loss of deposit or whatever it is) or just let him walk. Be sure that you have a clause that clearly indicates that visitors can stay for X number of days and that any stay longer than that requires LL approval. This will eliminate the possibility that he brings her in under the radar (do drive-bys frequently IMO).
Probably best to just walk away and find a reason to deny, but I can't say that's the best thing to do necessarily. If he was good before he should be good now. Maybe look at it the situation as if they were a couple. Would it matter that they had two kids then? Etc. Good luck. I hope it turns out well for you.