The two previous posters pretty much covered what I was going to say. My horses are both fairly “hot”, and I have found that having them on 24/7 turnout, not much grain and no alfalfa have helped a lot. I also vary their routine, so they don’t get hyper about being ridden. If none of the suggestions work for your boy, you might need to find a horse psychologist or a different vet. Tranquilizing should only be used for a specific, short term issue (such as a horse who would rather go over backwards than be clipped), not something that seems to happen randomly for no apparent reason.
It is never the horse's fault