You should be concerned.  That is not normal for the CO. They may have Pano - growing pains - but an adult dog should not knuckle over unless there is a problem. I suspect that you checked the feed and did a general feel of everything.  Take him to a good vet and explain what is happening.

You may have to get x-rays.

This is a large breed and they are prone to some problems but this is not normal.

Regards,

Gary