Does your horse have shoes? If he doesn’t this could also be it. He might be getting something called gravel which is when the sand in the ground gets up into his feet and causes abscesses. Your farier should be able to tell if this is the cause. I live where the ground is pretty sandy and I have to keep all 4 shoes on my young horse who otherwise has beautiful, hard great feet, because without them he has nonstop abscesses, even multiple at a time.