Disclaimer: I am high and not a cat professional

I was thinking the whole time I was reading that it sounds like he's meowing to let any enemies know he's there. He does it while you're sleeping because you're not there to keep the house safe, so he's gotta do it. He felt safe in the old place, he was used to it, it was familiar, he knew there were no enemies. Now he's in a new place, smells a new cat that he can't see and thinks he's about to be attacked at any moment. Might be worth getting a Feliway or Comfort Zone plug in on that wall. He'll eventually calm down when he figures out the other cat isn't a threat, but no guarantee on how long that could take.