A combination of diet and exercise may improve your horse's condition. The horse needs protein to build muscle and lots of work to bring it along. Trotting and lots of it. Up hill work is good too. There is no quick fix for conditioning, it takes dedication to a consistent program to see results. This doesn't mean that you cannot mix it up and come up with new exercises to keep his mind sharp, it means you dedicate enough time every day to get results. As he gets in better shape, the work out needs to intensify if you want to see results. Also be sure to work the horse the same amount of time in each direction if you lunge or roundpen. Working him in only one direction will result in more mass on one side than the other. I prefer to ground drive and take my horses out to trot them, because theybget bored with the pen, and youngsters joints take a beating working in a small circle.