I would contact a vet and see what they say. Do you know if the horse is hypp positive?
I would be very careful about what food you give him. Avoid beetpulp with molasses as it could trigger an attack.
Make sure he is getting less than 1% of potassium daily. http://www.bringinglighttohypp.org
^ that's a really great site for managing an hypp horse.
Like I said, I would contact a vet or someone with knowledge of hypp that can help you find a diet that will help him gain weight.