If he eats like a horse, that's good. If he eats like a pig, that's not so good. Horses eat a lot, but they don't devour their food. And if they had napkins, they'd use them.
Horses are big and they are supposed to eat a lot.
First we need to know how much nutrition our horse needs. Both are high in protein. You have to make the determination. Here's a way to start. Buy good quality grains from feed dealers and producers you trust.