I would suggest a weight builder....there are many different kinds on the market.....but we use Cool Calories, it is the only weight supplement that worked on our emaciated OTTB.
But EVERY underweight horse needs to be on probiotics!!!!! I swear by probiotics. When we put our OTTB on them, he started plumping up really fast.....now he's a little too plump. lol.
Probiotics come in many different brand names, like Probios, the most popular one. We use SmartDigest by Smart Pak.....
But your horse SERIOUSLY needs to be on probiotics. Try it, I can guarantee you won't be disappointed.