What the others are saying: Muzzle him. Your horse will not think you are “mean”, or hate you. And you might save his life! Obesity is a major factor for laminitis, founder, and other problems.
I have an extremely easy keeper, and he has a muzzle on all summer. He eagerly sticks his head in it every time I put it on! The Mrs. Pasture cookie might have something to do with that That’s the only time he gets a treat – he eagerly looks for the grazing muzzle when I’m getting ready to turn him out to pasture. I use the Best Friends Deluxe model.
You might also want to consider taking your horse off grain, if he’s getting a typical sweet feed. It will not be helping.
FWIW, in addition to the muzzle, my horse gets his hay rationed (weighed), and in a small-hole haynet when he’s in a stall. He also gets NO grain at all, just a little Triple Crown Safe Starch chopped forage, once a day, because he won’t eat his supplements otherwise (if it weren’t for the supps, he’d get only pasture/hay). He gets ridden 2-4x per week. And he’s right at a 5 on the Heineke scale. It takes a lot of work to keep him at a decent weight!
So, suck it up, and get a grazing muzzle on your boy. It’ll be an adjustment for him, and it might be a battle at first (I went through 3 muzzles the first summer), but stick with it, because the alternatives are Not Good. Also, as others have said, turned out with a grazing muzzle is a lot healthier, mentally and physically, than being turned out in a drylot, or kept in a stall 24/7.
Proud Partner of Yankee Allstar,
Rocky Mountain Gaited Horse