You may have to try different products and see what works for him. He may be feeling nauseous (although horses can’t vomit with their one-way system), which is why his sense of smell is heightened. It could be anything that is causing that but because he has some medical issues and is/was treated for Potomac Horse Fever you will have to see what works. Some horses like Apple Sauce or Molasses and some don’t. Do be careful with the amount of sugar in take from them. You could try adding a cup of Bran to his feeds with the apple sauce and water. It is very important that when feeding Bran, one needs to always add water to the feed when feeding Bran. Make sure the entire feed is mixed up and is wet (not soggy like a mash). I understand that it is difficult to “cater” to each horse at a livery stable so if you supply all the necessary materials, that might make it a little easier for whomever is feeding. Supply a spoon, bottle of water, bran and other supplements in the feed room so it is all right there for the person. I would also try introducing the supplements gradually again as not to overwhelm your horses senses. Feeding Alfalfa hay/cubes is very rich and “heavy” in the horses system, so if you need to feed additional hay, try Timothy which is less rich and dense. Not knowing your particular situation, it is difficult to give suggestions but I would set everything up and make it easier for the people that feed so your horse gets it all without resentment from the people doing it. Talk to the stable manager or the person that feeds and explain what you would like to do and see what would make it as easy as possible for them. I hope you find a solution.