I had one of these. My vet had treated him for previous owner, so I asked her what to expect. She said he had ALWAYS been "rangey". He had a corneal ulcer that would not respond to various topicals, so we ran a course of Tucoprim through him, the another to finish it up.... MIRACLE!! In 8 weeks, he turned into a Breyer horse....he looked plastic. The vet said she would NEVER had dreamed he would look like that, ever.
You might have the vet run bloodwork. I fed my horse Equine Senior, a little oil, and Vita-Flex Accel Lifetime. Mixed grass hay, nothing fancy, and maybe a bit of alfalfa, if I had it.