I see you have a TSC....
get him alfalfa pellets, to replace some of the "sweet" pellets, at least, needs soaking, tho. Then, for coat, skin, to add yeast, probiotics, fat and some vit/min, add Omega Horseshine, or the Dumor equivalent. With a cup a day, the bag should last two months, at least.
The slowfeeder is very good, needs to be full at all times, when he is stalled.
Look into a senior pellet, Nutrena Life Design Senior being a good one, to help him gain the missing pounds, then you can switch back to the current feed. Senior is easier digestible and has done wonders on my horse. For salt, the only salt lick they readily consume is the Himalayan one. Pricey, but worth it.