How much has built up? Can you post a photo of him? If there is not a lot of buildup, you might be able to rinse it gently with some saline and a q-tip. If it has built up a lot, you might want to have a vet take a look. You'll also want to find the cause for the buildup. It could just be some mucous buildup that goes away on its own. It could be caused by irritation like a piece of shed skin or sand/dirt caught in the eye causing irritation, or it could be the result of an infection.