It sounds like an allergic reaction to something. Do you have benadryl? Diphenhydramine is the generic name. It is an antihistamine and will help! Here's the dosage info .....
Is he breathing OK? Acting OK? The swelling is only on one side of his face? Did he get bitten by a bug, stung by a bee? It could be any number of things.
It's important to get the benadryl into his system right away and then call an emergency vet.