You say he was recently dewormed, but with what? Parasite resistance is becoming a big issue--strongyles are resistant to fenbendazole in most areas and to pyrantel in about 50% of the areas tested. Also, are you deworming for tapeworms?
When did he last have his teeth checked?
What is the quality of the hay he is eating?
What kind of feed is he eating--pellet, sweet feed, etc?
Does he eat eagerly?