How often do you run barrels? If he is really good at it, do you need to practice or are you referring to competitions? It might be that he is tight because it is just too much, and he needs the more relaxed routine you have mentioned for general every day activity. Relaxed in the sense that he is not going to be working such a strenuous and tight routine – obviously you aren’t just going for a stroll in the park! Have you mentioned this to your vet? Is it possible that he has a strained muscle or something?
It is never the horse's fault