I sure hope it isn't cancer. You don't know what's wrong with him, or what kind of cancer it is, if it is cancer. I suggest holding off those thoughts until you have a diagnosis.
If it is osteo, Jen is right. The longer you wait, the greater the chance of a broken limb. Also, if it's osteo, he'd be in pain, even if he's not showing it. You wouldn't be doing him any favours taking him to the walk.