Have you contacted to company and asked them if those weight recommendations would cover ponies? With his breed, I would perhaps err on the side of cautiousness and just got to the ration balancer and fat supplement if he really doesn't have the stomach size for 2# of grain a feeding. How big (height weight) are you expecting him to mature? I know lightishpony... 12ish hands? 500lbs?
If he really needs the calories, see if he will eat 2#. Make sure you WEIGH it. You run the risk of not meeting his vitamin/ mineral package and Ca/ph ratio otherwise.