I don’t know about the legal side, but the ethical thing to do might be to return the check to the insurance company and have them issue it again, only in his name instead.

The money came out of his escrow.
The check has only been generated because he’s decided to change company/policy on a property that is now in his sole ownership and has been for a while. You’ve been nothing to do with the house since March. Even before that, it sounds as if you handled the paperwork, but he paid for the actual policy?

I think it would be dodgy to hold onto it or to try to cash it…Why risk being charged for theft, potentially?