Please have your piggy seen by a vet. I agree with Freela that it does not look your typical fresh bite wound, more like a little abscess or cyst has burst. Please disinfect it with saline solution. In an emergency you can make it yourself by mixing one teaspoon of salt into half a pint/250 ml of boiled cooled water.
If you are worried about fighting, please take the time to read this guide here; it tells you in detail what you can do at any stage of things not going well. Neutering is useless; it doesn't change behaviour or personality (testosterone is still prduced and liberally sprayed all over the place, as my neutered "husboars" amply prove when they stink up the place). Neutered boars also still go through all the stages of a teenager the same as an unneutered boar.
If yours don't get on, then there are other things you CAN do:
Boars: Bullying, Fighting, Fall-outs And What Next?