Did you have a blood panel done? It would not hurt to get some "iron" into him, Red Cell, Lixatinic something like that to give him a boost.
Make sure you are weighing your feed, not just scoops. Sounds like you are, but use a scale and make sure exactly what you are feeding.
When we were trying to get our rescued Bones weight gain(he was a 1.5) he was getting 14 lbs of Senior feed daily, broken up into two feedings. I actually got a scale and measured it, not just used scoops or quarts. It was amazing, because 7lbs of this feed filled up almost to the top one of those smaller colored buckets. I was surprised how much it actually took to weigh what I wanted to feed him.