My tb gets the following twice a day:
~10lbs of orchard/pasture grass hay
~5lbs of alfalfa
3lbs of Strategy Healthy Edge
3lbs of COB
1lb of Elk Grove Milling Stable Mix
+ his supplements
All this just to maintain his weight. If he was in real work of any sort he'd probably have to get that by half again or more. It sounds like the work your horse is in is easily burning off the food he's taking in. Some horses, regardless of breed but in my experience especially thoroughbreds, need WAY more food than people expect and they end up starving (for lack of a better way to put it) because no one realizes their real needs.
Just because you are boarding somewhere doesn't = an excuse for not knowing what, how much and how often he is being fed. For your horse's best interest you need to find these things out and work with both your barn owner/manager and your vet to come up with an appropriate diet.