Sounds like he has a bad arthritis problem in his knee.
I had what I thought were healthy legs and was playing competetive baseball until 3 years ago and was diagnosed with degenerative arthritis in my knee.
3 surgeries later, I haven't played an inning since.
Mine is in my left knee and it always hurts when I play golf. I can see how the torque in his swing would be extremely painful for him.