Get something with ivermectin and praziquantel as the main ingredients. Pyrantel pamoate isn't going to get rid of tapeworms. You should be rotating your dewormers. Young horses can have very heavy parasite loads.
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.