This is a real problem. It most likely happened a while back, maybe even at birth, and has just now gotten bad enough to be noticed. He needs an antibiotic. I would take that photo into a vet, and see what they say. E-mail it if possible. Give the age, etc. Ask if you can treat it in the meantime with a topical wound spray for cats like found in the farm supply stores. even a half water half solution of hydrogen peroxide. He definitely needs to be seen or at least have some kind of treatment. if he is nursing that is a good sign. it almost looks like a burst abscess, or he was accidentally bit while the mother was trying to chew the cord.