Why is my cat's cheek swollen?

It is possible that your cat may have a abscessed tooth and still be able to chew on the other side. Any swelling is a red flag and needs a vets attention. He may also have a bit of infection from the scratch not visible to you at this time. He is also drooling in excess and that also indicates a mouth problem. Personally I would book an appointment on Monday to have him checked. I hope this is of some help to you.