Definitely I'd go with a foal grain/supplement. The purpose of these is not to feed huge amounts, but rather to use to ensure a balanced diet and that nutrients are available to the growing foal particularly during periods of rapid growth.
If the foal had access to miles and miles of top quality pasture, where he could eat from a variety of grasses, legumes, herbs, seeds etc... then he likely wouldn't require anything else.
However, the truth is, each individual is different and each body utilizes nutrients at a different level of efficiency so best to cover your bases to help prevent issues down the road.