There is an ointment you can get from your vet too which will help ease the pain for the sheep. It's so long since we've neeeded to use it that I can't remember the name. When you put the ointment in the eye, wear disposable gloves and be very careful with your hygiene as some conditions can be passed on to humans.
It sounds like pink eye, which is properly called infectious conjunctivitis, but it could also be due to an eye injury. If your wether is a multihorned sheep then the problem could be SUED but I won't elaborate if it's not multihorned.