Get teeth floated, deworm him thoroghly (or do a fecal for worms), and increase his hay. He should be getting double that hay, or at least another feeding of 2 flakes at lunch. He should be getting 6+ flakes of grass hay a day (20-30 lbs).
Have them feed just 1/2 scoop of the barn feed twice a day and add 1/2 scoop of alfalfa pellets and a vit/min supplement.
Take his weight with a weight tape once a week at the same day/time and in the same spot. If he's slowly gaining after 30 days, then you're good. If he's not gaining, then add 1 cup of flax meal to his diet. Most feed stores carry or can order generic ground flax in a 50 lb sack. I pour it up into two big white buckets with lids sealed tight (it can go rancid if moisture gets in). Feed 1/2 to 1 cup per-feeding. Decrease the barn feed to just once a day 1/2 scoop and increase his Alfalfa pellets to 1/2 scoop with the barn feed, and 2 scoops when he gets no barn feed. He WILL gain weight on this diet!
Once he's at a good weight, keep him on 5-6 flakes of hay a day, 1/2 scoop of barn feed and 1/2 scoop of alfalfa feed AM, 1/2 scoop alfalfa pellets PM, with a vit/min supplement in one of the feedings. You can feed just 1/4 cup of flax to keep his coat shiney and add Omega 3's to his diet.
If he sifts out the powder (vitamins and/or flax), add a little water or corn oil to mix it all together.