No idea haha, I've never heard anything like that, especially since the stomach and knee have no relation. However my training is for life threatening situations anyway so I would have no idea about this. Now if he was in a car accident or having breathing difficulty, trauma, spinal injury, ex. I could help. 
He needs to go to a specialist for joints and such. Possibly get an MRI or X-ray taken.
So after going on about how you're a trained medical professional, the answer is the same. Go see a doctor.
