Sounds like he's got a high sugar diet, your farrier was right, it is bad, but cutting it isn't the answer, they're not eating the dead grass (which is bad too) they're eating the yummy fresh stuff underneath.
Unfortunately it sounds like her needs to be put in a field with little to no grass you can try keeping him on the hay he's got, it's better than the grass, or giving him soaked hay cubes (timothy alfalfa mix) which most horses love. No horse would eat hay when they could eat grass so he's gotta be in a position where he doesn't have a choice. I'm surprised your vet didn't mention this, it's fairly typical for cushing's horses and limiting grass intake, especially the short stuff, is a good way to reduce the extremes of Cushing's symptoms.
It's like giving a kid candy all the time.
It stinks we can't spoil them rotten :(