I know you might not want to do this but it might tell you if he's hurting somewhere... Let him play. Let him run around and play with Cooper. If he plays hard and starts limping a lot, bring him over and poke around. You might find the spot that makes him hurt. He's a dog, he's really intune with his body, moreso that we humans are. When he hurts, he'll let you know.
I had to do this with my Sheltie when he was young. It's just a suggestion.
