Have a vet check him out. Teeth first. If he cannot chew properly he cannot utilize his feed, no matter how much he gets. He might have worms.
Hay in stall....2 lbs of hay IS hay. But nowhere near enough. Pasture is just now starting to come up. Winter grass has next to no nutritional value.
He might be a hard keeper. Other horses do just fine on hay and grass, hard keepers need more/ better feed.
To give you detailed help we need to know what he currently eats. And how much. By the lb, not flake and coffeecan as measure.
Parallel to that schedule a vet appointment and have him checked out.
And use the info affored has given and read all you can to become an informed horse mommy