I'm so sorry you now have this to worry about. I hear you on downsizing, but I just can't I don't think...Dakota was only 60 ish lbs. And it was really difficult to carry her. More so because she was afraid and in our case a lot of pain interfered as well. And I have only 3 steps. I can't imagine a whole flight.
Anyway I agree for sure that a break is in order, just how long? I think I would do a day. Then start or try to start a new day heading right in to what you want to accomplish. Lots of cheerfulness in your voice as you coax him too. I think you'll be able to get him to relax a bit. Hugs for you both.