First, it depends on hay a lot.
My horses run back to the hay rack after being on pasture for couple hours. My neighbor's horses who are on pasture a lot don't gain that much weight either. I'd feed him grain at least twice a day (a scoop). Plus may be add weight build in it and veggie/corn oil for sure. That's how I brought weight on mine (she was much thinner).
He has thin neck and cow-hocked. Lovely color, but I'm sorry he's not breeding quality (yes, I know he's baby, but the overall confo problems don't go away with age).
Iridehorses, that's HORRIBLE! I'm very sorry to hear about her! We always learn the hard way... :(