I'll keep my fingers crossed for you. If it 's eating than he obviously isn't too stressed out. You rays are always very heathy and not to mention amazing looking. I have no doubts that under your care he'll pull through.
What is your filtration intake like? Overflow box or Canister? Just covering all the bases, and I know this is a stupid question for an advanced ray keeper.. just curious though...
Not as much worried about water quality as I was thinking perhaps the ray got curious with a powerful canister filter intake, and could of had some 'suction' time on his underbelly, which developed into a sore..