It could be a strain/sprain that he's re-injuring when he's running around. I see also that he's on calcium supplements - sometimes these can lead to growth-related bone/joint pain like panosteitis, OCD, or HOD, so keep an eye on things; if he starts limping/acting sore that's not associated with exercise, or if the lameness shifts from leg to leg (or if he becomes feverish, lethargic, and lacks appetite), suspect one of these.
http://www.labbies.com/dysplasa.htm
http://www.labbies.com/dysplasa.htm
