I have a similar problem with Bailey. During the spring (now) I have to watch his grass intake as he blows up like a balloon. I do use a grazing muzzle which i find helps a lot. he can still be out in the big field, playing, but has limited feed.
I also lunge him in the round yard, although I haven't done that since last year. You must build up to it slowly though.
Are you able to get your colt out on walks...like taking a dog for a walk?
I know that Bailey and Willow play HARD out in the field and will gallop around for long periods, stretching their legs and being silly. Is your colt out in a field with friends so that he can play?
Another good idea is to soak any hay you feed to leach out the sugars before you feed it.
A combination of the above got Bailey looking nice and trim and in a much healthier state.