I'm no horse expert, so this is not an educate answer!
I used to own a horse when I ws younger and he was probably a pig in a previous life.
It seems a shame to muzzle a horse during the day if it is to be kept outside. I shouldn't imagine that grass contains that many calories to really affect his weight, also, imagine how you would feel if you were on a diet and stopped from eating healthy food. I'd say that he is getting too much food in his bucket, or maybe the wrong kind of feed.
I would contact your vet and arrange a proper diet plan. Perhaps you could put the horse out on loan to an eager teenager who would love riding him.