I wanted to add about the oat hay. As long as he has good teeth, an oat hay mix is a great choice. If he has bad teeth, grass and alfalfa is best.
Keep us updated on how he’s doing.
My Frodo (pony in my siggy) has a pretty bad infection from his teeth right now and we’re not sure if he’ll pull through right now. So I know how much you can worry when something is wrong with these guys.