There could be a couple of things going on.
- I would think that he might have been bitten by an insect or stung by a bee. If that's the case, your vet can prescribe an antihistamine that can help a great deal with the swelling.
- He could have had a trauma that you cannot see. If you suspect this, you can flush the eye with saline and then dry it well. This will remove irritants. The vet might want to stain the eye and see if there is a scratch that is not visible even to the naked eye.
There are no over the counter meds that are safe for cats so be careful about giving anything without talking with your vet first!
Let me know if you still have questions.