Having Googled last night (always fatal when you're worried about health) it looks like it's either something in his eye, inflammation of the tear duct, or conjunctivitis. Apparently it's also a symptom of feline Herpes, but as he's an indoor only cat in otherwise perfect health I discounted that. He's now a bit puffy under his eye as well.
Had to be Boycat didn't it! The one who is absolutely terrified of anyone except me.
Good news at least Coastergirl 