As the owner/foster-er of quite a few senior horses, I’d also recommend a bute test if you haven’t already done something similar.
I know you said your vet has checked him out and his teeth and saddle fit are good, but putting him on NSAIDS for a few days and monitoring his behavior is a good way to tell if there’s any low-level pain that is contributing to his actions and attitude.
Whether he improves with the meds or stays exactly the same, that’s more helpful information about him for you.