I would really work with a vet first before throwing hay and grain / oats at him. Why is he underweight?
Has he seen a vet?
Has he been wormed since you have had him?
Blood levels tested?
Teeth checked / floated?
I personally would feed 1/4 flake of grass and 1/4 less of alfalfa (it can run hot, and depending on your weather, if he sweats, he's going to be lose precious fluid levels. TBs run hot anyway, so just watch him) 4 small meals a day. I would feed it completely grass at night since grass hay takes longer to digest, it will be in his system longer. Keeping him physically warmer at night, and keeping him "full" through the night.
The worst thing you can do is dump a ton of food infront of a starving horse. You are asking for more trouble, and it wont do him any justice.
Take it nice and easy, but steady. He will eventually pick up the weight qucikly, and in a healthy way.
"what causes an echo - the persistence of sound after the source has stopped. "when can you hear an echo?" when it's quiet and other sounds are absorbed. when it's quiet, i can hear my mother's echo still."
-For one more day