What to feed my horse?

I have a 16yo 16.3hh thoroughbred gelding who is schooled in mostly flatwork for about 40 min each day 6 days a week. I want to give him a bit more energy to be able to carry out the work efficiently but do not want him to bulk up too much. He is currently having green chaff, white chaff and pellets in the ratio 2:1:1 but i want to replace one of the green chaff with something a little more starchy perhaps. He has plenty of gr to eat too. Are there any suggestions as to an energy giving feed such as meusli or something?