I can't stand eye problems either, Lucinda. Nothing makes me squirm more! With it clouding over suddenly, and being sunken, it certainly sounds like an injury. Most likely to be a hay poke, as others have said, though other issues can be the cause. Optrex Infected Eyes are a good over-the- counter solution, since they contain Chloramphenicol - what most vets would presribe - but obviously a vet check would be advisable to ensure it is just a bit of a poke.
Willa got bitten on the eye when I first got her - the first eye problem I've had to deal with. Hers clouded over, was bloodshot and you could see a white scuff mark where naughty miss Meghan nipped her. She was back to normal within a day of starting the Optrex ointment. Willa's injured eye was very sensitive to light; keeping the room a little darkened will encourage the pig to open the eye a little and let it heal faster.
Good luck with him, let us know how he gets on.