If it is coming from his rear legs then I would agree to have an x-ray done. I have a 2 1/2 year old boxer and when he was just over a year he would do the same thing after running a lot. We took him in and they took an x-ray and the ligament that extended was over his knee cap was stretched, so then his knee cap would move out of place. So we had to put him on glucosamen (sp?) and if that didn't work then he would of had to have surgery.
I don't want to diagnose, as it's not my place, but you may want to have an xray done and have them look for ligament tearing, or cartilage damage.
Same thing happened to my English Mastiff, on both sides at the same time (she tore her ACL and MCL, rear legs), and had to have (2) separate surgeries to repair them.
Talk to your vet and see what they think... I'm willing to bet that if you call them, they'll say that an xray is more than likely the next step.
Good luck.