Building my horse's muscles?

I have two horses, one who was gelded early and one who was gelded when he was young but later than the other. The one gelded later had a chance to fill out, he has a lot of muscle. The other has very skinny legs and my parents always say he has a belly. Since he was gelded early does he have a chance to gain muscle and lose that "belly?" And are there any good techniques to build up his muscles quickly? i don't get to go there everyday.