The fact that he carries 10 to 15 lbs too much weight on fight nights doesn't exactly help his gas tank either.
Yes, he needs to be closer to 230 with his narrow frame. Being leaner and looser would help him immensely with his stamina and general fluidity, though his power would suffer. It's a necessary tradeoff, however.
I think coming over to America to fight at the MSG just took its tole on him. We have to remember he is so used to fighting in the UK where he sees it as his Lion's Den. Coming to the US to fight at the MSG where it has been many great boxers 'Lions Den' may have just swallowed his confidence. It wasn't his backyard anymore. He looked like a rabbit in headlights. Regardless of the loss he needs to step out of his comfort zone (The UK) and continue to fight in the US like Tyson Fury is doing. He can only grow in confidence from this loss and he did lose to the better and more confident fighter. We are all built mentally different and this time Andy Ruiz Jr was mentally stronger than AJ on the night.