Keratoconjunctivitis Sicca, an underproduction of tears that dries the eye and causes redness, irritation and inflammation. A sticky ocular discharge, clouding and scarring of the cornea may lead to blindness if the disease isn't treated. One eye or both may be involved.
Uveitis is often diagnosed as Conjunctivitis, but it's actually the inflammation of the inside of the eye. You may see clouding, redness or bluing of the whites of the eye. Many times it's the result of an overactive immune system, but may also be the result of injury/trauma, disease or tumors. It's sometimes a persistent condition that may need lifelong treatment of steroid drops or oral meds, anti-inflammatory meds/drops and Atropine to reduce scarring along with pain relief.
There are other, probably lesser likely possibilities - far too many to list - and some pretty complicated to treat, so let's hope this is something as simple as a hair got into his eye and caused some bad irritation.
It's a good thing you noticed this early - by getting him seen quickly you have a much better chance of a quick and successful solution.
Keep up the good work!