It's called a corneal ulcer. It's very common. They are extremely painful which is why he is not himself.
Lady had one once and it healed with no problem. The danger is with not getting medical treatment right away. Since you took him to your vet and have drops, it should heal without problems. If it doesn't, your vet will probably want you to see an opthamologist.
Here is some information.
Corneal Ulcers In Dogs and Cats And Their Treatment In ThePetCenter.com.