Ulcers (either gastric stomach or hind gut), insulin resistance, equine metabolic syndrome, and cushings were my thoughts.
And to compound matters you can have ulcers with any of the other three and you can have ulcer or IR or EMS along with Cushings.
The horse doesn't necessarily have to have a Yakky-looking coat to have Cushings.
My EMS horse went from being an air fern to a hard keeper and lost a tremendous amount of muscle mass. But, yes he does have the cresty neck and the fat deposits on his rump.
I would get him completely of grains and feeds with grain in them, just in case he has a metabolic or digestive issues. You could put him on a vit/min supplement, mixed with either equine rice bran or pelleted beet pulp.
If you have another vet in your area, I would get a second opinion since your first vet isn't concerned. You've got enough experience that, if your gut is telling you something isn't right, then it isn't right.
"always go with your gut" -------------