When we were little and had company, we were evicted from our rooms and the company got to stay in them. It was couches or the floor for us (or the guest would stay in the nice ‘girls’ room and we’d oust our little brothers from their beds).
I think your MIL should get the nice guest room and your son should make due with an air mattress, an AIRB&B, or whatever you can figure out. Our neighbor does airb&b and we’ve thought of that as overflow, but for now we just make do with couches.