As much as he wants. They should never not have hay at their disposal for the digestive tract to work properly.
Between 8 and 12 kilos according to the foal.
As said Mark, it depends of the area he lives in and if he eats pellets or not.
It is about 10 kilos per day. But consider the place where the foal lives, the quality of the grass if he is in a field or paddock and so on.