Flutterby,
He could of injured his acl. When Libby was about 4 months old she tore hers the first time running around like a loon. At first I thought she was having hip problems because she never yelped when she first hurt herself, but they x-rayed her hips and said her hips were excellent but that she needed surgery for her ACL. We had to keep her "quiet" for 8 weeks. She couldn't do anything, she had to be on a leash to go potty and my hubby built her a ramp because she couldn't use stairs. Still with keeping her quiet at her 8 week check up we were told the surgery failed so they did it again that time I told them to put a cast on her leg. She will still limp ecspecialy after she plays. The vet did say that we will need to get a TPLO done soon on her which is VERY VERY expensive. I would take him to the vet to have it checked it could be that or something minor, but if it is the acl or ccl if its left untreated he will get arthritis. Pop back in the hip thats kinda funny, my mom says things that make me wonder also.