I would recommend the Tribute Essential K for your horse. It is a ration balancer with a higher protein level, and is great for ensuring they get all their vitamins and minerals, and the higher protein content helps with developing muscling.
I would sub out the complete feed you are giving him for the Tribute Essential K with the beet pulp. It is a great, quality product and my horse did wonderfully on it. The feed rate is quite low, so make sure you read the bag for feeding instructions. Your horse would probably get about 1.5-2.5 lbs per day, roughly.