I would 100% recommend Seminole wellness senior mix, alfalfa pellets, and beet pulp pellets, and have all of those soaking for at least 15 minutes with warm water (it doesn't have to be warm, but it feels better for my horse). The senior mix is so he can get the proper minerals, nutrients, fat, and it helps keep fat on, the alfalfa pellets are for extra nutrition that he may be lacking if he isn't getting any hay, and the beet pulp pellets are great for keeping or getting fat onto your horse. 100% recommend this formula, and to make it into a bran mash everyday. As for supplements, I don't have an answer, but I would say to put in on Probios which is very good for his digestive system and some electrolytes to encourage drinking. You can also try DAC oils, and supplements. I had a 20 year old pony who was still running 17 sec barrel patterns who was on DAC supplements, DAC oil, and Cosequin.