How is the pony's body condition? It would also help to know when he foundered, and if the vet knows why. Is he IR or Cushings? Did he get in a feed room?
The following advice assumes this pony is overweight and chronic/IR, something alongs those lines, as that's common with ponies:
In that case he should be getting 1.5-2% of his ideal body weight in grass hay, soaked and rinsed unless you know that the NSC level is low enough. This horse probably should not have access to pasture, or needs to use a grazing muzzle. He probably does not need any hard feeds, but needs a multivitamin or ration balancer since he cannot have pasture. Magnesium and chromium can sometimes help these horses, as can ground flax seed. I feed these in a small scoop of timothy grass pellets (about 1/2 lb).
Do not under any circumstance feed anything with molasses or sugar, corn, oats, or other grains. Not even treats. If you must give treats, I suggest hay cubes or special low NSC ones, like Skode's