We see a lot of ACL tears (full and partial) at our clinic, and some of our doctors do surgery to repair them. Partial tears can go either way - they can repair themselves, or they can fully tear. The *only* fix for a full tear is surgery. And if one knee goes, the other is likely to go, too. Some pets will have full or partial tears and never show any clinical signs. Animals are so amazing at hiding their ailments...

I'm hoping, for your sake, that it is just arthritis. But to err on the side of caution, I would try to limit his activities to preserve the ligament in case it is a partial tear... at least, until you get a definitive diagnosis. Forgive me if you've already stated this and I misread.