Chill out. Offer him a cold teething ring or a frozen, wet washcloth to chew on. Or give him cool water in a sippy cup or bottle. offer a massage. Rub his gums gently but firmly with your clean finger.
Try medication. Ask your pediatrician about using a pain reliever, such as ibuprofen (if he's older than 6 months) or acetaminophen, if your little guy is really miserable. A topical pain relief gel will be washed away rather quickly by drool, but can provide some relief.
Distract him. Read and reread his favorite book to him, as long as it seems to make him happy and keeps his mind off his kisser.
Take it easy. Give your baby extra cuddles and attention. Don't try any cry-it-out sleep methods right now. He needs lots of hugs and kisses, and a snuggle in your arms can take away the ouchies better than anything else.