Arthritis is a condition that can flare up, and also "go away". I'm not sure you can "manage" it.
So it's not some big shock that he is running like the wind.
I'm also not sure if "resting" him or splitting reps has anything to do with reducing flare ups. I don't know if rest helps or if it just becomes painful on occasion regardless of use.
It's going to effect his career negatively..........which sucks because I really like him not just as a player, he's seems like a great team mate and person.