My horse is overweight and a big eater, what the best horse feed to put him on?

i'm assuming you're from america by the things your horse is in training for so i can't really recommend any brand specific horse feeds because I think they might be different there however i can recommend maybe a low sugar, high fibre diet. if you don't want to feed him less maybe try exercising him more. have you thought about maybe lunging him if you don't have time to ride or riding for longer uphill - obviously working slowly to build his fitness. what i tend to do as well is just feed pony nuts with lots of carrots and apples and possibly a tiny bit of chaff and then mix in a good dose of vitamin powder (horse dependent, i used to use ones for the older generation) so he'll still get the nutrients and still be in his feed routine but wont be getting any of the calories. balance this with a good exercise program and he should be fit and slim in no time :)