How deep is it? Any puss? Can you post a good quality picture of it?
My first thought is an abscess that has broken open. An abscess usually happens after a puncture wound gets infected but heals over on the surface. This can happen during a fight with another animal or even just play fighting with his brother. They often just break open on their own but sometimes are deep enough that they need to be lanced and drained. Many will heal on their own, but sometimes antibiotics are required depending on the severity.
In some parts of the country, bot flies have also been known to cause open wounds, but I don't have experience with that and have no idea if it's even the right season for that sort of thing.