I bet that he has mites. Sores, scrapes, cuts, hairless patches on a guinea pig often are signs of mites. Read about
mites at www.guinealynx.info . You don't need a vet visit, you can treat them yourself. If you have any other guinea pigs, you will need to treat them, too.
As for what to put on the cut, it's best not to put anything on it. Guinea pigs can get sick from neosporin or other antibiotic ointments (when they lick it). Just make sure it stays clean, and watch for swelling (abscessing).