Does he love running or does he think he is running for his life? How can you be sure? Just because he does it at home as well as at shows doesn’t mean he isn’t stressed. To calm mine, I have just periodically got on them bareback with a halter and lead (in the ring) and just walked. Then, I get off. If I keep it random, they never know if they will be working or just ambling around (and I do the amble thing with full tack also just to confuse them). They are less likely to act like loons when they don’t know what to expect. Of course, mine are not every horse – they are special loons, but it works for us. Generally, I find that treating behavior issues with diet does not, in the long run, solve the problem. I would also consider “high strung” and “nervous” to be essentially synonymous. From what you have said (and I have nothing else to go by, not ever having seen your horse), it sounds as if he is anticipating what he thinks you are going to do, without enjoying it. Why is he overweight and out of shape?
It is never the horse's fault