It sounds as though your little one may have lp. Lp is measure in grades of 1 - 4, 4 being the worse. It is based on the severity and how easily the knee goes back into the groove. Athogh a dog can have a higher grade, it does not necessairly mean he is in pain with the lp.
Some on here have said that their dogs have a grade 3 with no issues at all while others have needed surgery for a grade 3. In my case, Hot Rod went from a grade 1 to a grade 4 in months time and was in pain. Maggie May was just diagnosed with a low grade 2 and I was totally shocked as she's never exhibited any sign of it.
They are both taking supplements for it; we give them Dasuquin. Cosequin is another supplement that you can give. Exercise is good for them. What you want to try and do is lessen the motion of jumping. Stairs and ramps would be good to look into.
Is your little showing signs of pain?