How can I get my horse to gain weight quickly?

Alright, so my AQHA Gelding is just a little under weight, no big deal. But I really want him to look like a foundation AQHA, he has the muscle, bloodlines and build for it I think he just needs to be fatter :P (don't freak out his ribs aren't even showing!) But I wanted to know if there's anything I can do to help him gain a lot of weight in a short amount of time. Like grazing every day (he does just not a lot of it) Or if there are any cheap supplements? Anything helps!