Bewteen those 3, I'd go with Safe Choice or Life Design. I've heard good things about Nutrena, and Safe Choice in particular. I'm a little biased, though, since I know nothing about DuMor.
Enrich32 is a ration balancer- meant to be low calorie- and it sounds like your horse needs a little more than that. You can also try adding some high calorie forages to his diet and stay on E32 if that sounds interesting to you; there are definitely health benefits to being on a low-grain or grain-free diet. Rice bran and/or beet pulp are common choices, and you could also try alfalfa (cubes, pellets, or hay), oil (fish, flaxseed, soy, vegetable, or others.), Cool Calories, etc.