So, windpuffs them selfs are not big deals, but they can mean that a horse is over using their legs. The windpuff is the bursa sack stretching and a bigger secretion of the synovial fluid which lubricates the joints which means he is being worked a lot, my young eventer gets them, not a big deal in its self as i said. BUT, there is a reason he is getting them, most of the time its because of hard work but sometimes they can signify that there is a problem. I have seen one horse that has them and he recently cut that legs while in a fit with a horse. When its in one leg and not the horse you should talk to a vet because they might be favoring the other legs and over using the one with the windpuffs. You should get a vet out there for a lameness check, he might need hock injections or something, maybe an IRAP for arthritis or maybe he just got in a fight and thumped his leg.
Hope this helps!