It's possible he has an eye infection. Is there any discharge or crustiness around his eye? If it's irritating him he could be scratching it or excessively washing it, resulting in the loss of around that area. Or it could be something like flea or skin irritation caused by mites or parasites (mange). It would be in his best interests and your own peace of mind, to have him checked out by a vet for diagnosis and treatment (cream for skin and/or antibiotics for infection). All the best.
