I have a large Thoroughbred, 17.2hh and medium to large boned. He lives in a dry lot with shelter. He gets free choice hay, but I'm afraid it's not enough to keep his weight up during the Summer when he's ridden. He's been rather "hot" in the past when I put him on grain. Any ideas of what to feed him to keep his weight up without making him over excitable? Low protein high fiber maybe?