if the water quality is ok then it is possible he damaged the eye somehow and it is a bit infected. Normally the eyes heal up quickly without medication but if there is no improvement in a couple of days, then you might need to invest in some broad spectrum medication or a general purpose antibiotic for him.
If you do need to medicate the fish then it is preferable to do it in a seperate container so the medications don't wipe out the good filter bacteria. But see how he does first, he will probably be fine in a day or so.
Some people use Melafix for situations like this. It apparently does not harm the filter bacteria but can help the fish.