This could be anything from an allergy to a viral infection. It would be a good idea to check his temperature morning and night, monitor his activity levels and appetite and contact the vet if anything looks "off".
FYI If it had been just one eye with a discharge, I would have said rinse it with plain saline like for contact wearers and see if it improves in a couple of hours. If not then get the vet out as damage to the eyes can lead to serious consequences in just a few days.
Licensed Veterinary Technician