'Often' a youngster with a big belly is too thin and needs more concentrate. Also if your hay is very high fiber that will make some belly on even a horse in good weight, but not a bloated look. The bloated look comes, basically, from malnutrition....think of the pictures of malnourished children in the news....skin & bones with big bellies.
Check the feeding directions on your concentrate and weigh your feed so you know that he is getting the recommended amount AND if he is too thin, give him more!
And good for you for recognizing something isn't right and asking questions! :aktion033:
Charlotte