If I were you, I’d get his metabolic profile done when the vet comes out. They’ll draw blood, and will be able to tell you how well his metabolism is working. They can test for IR, signs of Cushings, and thyroid function. Our Arab is an easy keeper, and put on weight at the end of show season last fall. We did a metabolic profile test (I think it cost something like 60$) and found out his thyroid gland is underactive. He went on a thyroid medication for most of the winter, and I cut back on his hay and beet pulp and he is looking great now. He is off the meds, and I actually increased his hay slightly because he’s being conditioned for show season now.
I agree with everyone that laminitis is terrifying and should not be taken lightly even if there is the shadow of a doubt. My neighbors nearly lost their horse to laminitis. You don’t want to let it get that far. Get your hay tested (also a very cheap test), soak it for now, and do not let him have any grass or any sugar of any kind (including treats). You’ll be glad you took extra precautions if he turns out to be at risk of laminitis, and if not, then there is no harm done. Good luck!