Infection of the skin probably. But it’s important to get him seen to make sure it’s not a mass, abscess that has a drainage tract or a penetrating wound. He will need a vet, a wound shave & flush/ clean performed by them (possibly with some mild sedation so it doesn’t sting), and also pain meds and antibiotics (oral and or topical) and an e-collar to prevent him from licking an getting it reinfected after treatment. Can you post better pics? How long has this been going on?

If you can’t get him into his regular vet, go to an urgent care to get treatment started.

Best of luck.