make sure he's not in his feild as much, as if he is, he is sure to be snacking... ALOT. and thats probably the main problem with his weight; and also another danger  of being out and snacking on grass alot, is he could possibly get laminitus! but if he isnt out all the time, it could be his diet? is he getting a healthy diet? the right nutrience! you can look up apropriate diets for the type of pony you have!

goodluck, hope he gets thinner!

p.s - do you excersize him reguly? because it might be that. make sure you work him harder but not too hard.