Hello,
It does sound like a scratched cornea. I worked for an eye Dr for three years and even with the sand out it is very painful if scratched. It does need to be looked at by a vet. Infection can set in very quickly. Keep a close eye on the eye and take him to the vet as soon as you can. Sometimes it is hard to see something like sand unless he is under a special light called a slit lamp which the vet should have. Good luck and keep us posted.