Do you have a Tractor Supply? They carry quite a selection of equine supplies. You could order online as well.
When my old guy started loosing weight I had to put him in a stall because he has a diagonal bite which causes him to take longer to eat. I also put him on more grain but I didn’t stop riding him completely I just rode for shorter amounts of time. I would talk to your vet, maybe there is beatpulp or weight builder you can put your horse on.
I would recommend to ask your vet what else to give as they would most likely have more of an idea on what to give according to your horse
Hope this helps