It looks like overall he could use more muscle mass in his hind end. Do you do low impact exercise with him or is it all running and jumping? Rafi was like that when I adopted him at age 1.5. I built up his muscle, gave him supplements for his joints and don't let him overdo the high-impact exercise and he is doing great now at age 7.5.
We walk regularly, but not for too long of distances. I can attempt to increase that as well as some hiking. What supplements do you give him?