you could find the answer when you get his hips/ellbows x-rayed, especially if he has been overweight as a pup. even ostechondros could be a possible explanation, but i won't say to much, since it can be that he has just been pushed to hard. limping can be due to a lot of differend things. but do ask the vet to have a look at his patella aswell when you are there. let someone show you how to massage and strech your dog after work out, especially streching is very importand after work out. i add glukosamine to my dog's food, try that aswell. good luck, i hope you boy will get better.