Dang, I wish that I wasn't the first to post. I am not saying this as a proffesional, but, it sounds like maybe a tumor? Malignant or not, it could bother his leg. Usually the dog shows pain with a tumor though. That is where I am confused.
Now, please, don't panic. It was just an educated guess. Also, a dog of any age can get a disease.

I will be keeping Duncan in my prayers, and please, don't get too scared, I could be COMPLETELY off.
Steph and Jes