If your trying to get weight on him try Cool Calories and/or pure corn oil.
I had a mare that I'd taken in and she was pretty skinny. So I started out with a 1/4 cup of corn oil for pure calorie intake once a day. I gradually upped the dose to 1/2 cup to 3/4 cup then to a whole cup of oil (to keep away the runs). The grain I gave here was whole oats and/or barley. Sometimes I'd get them crimped.
I read about Cool Calories. It'll add caloric intake without having them bounce off the walls. But trust me when I say it doesn't taste the way it smells. ;)
To me, it smells like Kool-Aid, but it is just bland as the day it long. LOL
There was something else I also tried, but it was more expensive and you had to feed WAY more than the Cool Calories.