I would get him onto Weightlifter. If you email them they will give you a free diet program. Its great for topline and putting on weight without getting them to fizzy. I highly reccomend it.
I had a gelding that was bought up on Weightlifter from a 2 year old till 5 year old. He won a few Res National titles are it all a credit to Weightlifter. Oh..and alot of elbow grease from me..lol...