I cannot stress this enough. If he has no fat elsewhere with a belly that big it's very likely your wormer isn't doing the job. Talk to your vet, get a fecal done, and if needed deworm him again. If he's free & clear alright you wasted $20, but you know that isn't the problem.
Check out this article: http://equisearch.com/horses_care/he..._guide_050808/
By just deworming with pyrantel pamoate you're allowing several species to continue to "hatch" in his body. You need to use a product that can kill them in their various stages.