There isn't a lot you can do given the location of the bite. It doesn't look that bad (compared to what some of my boars have done to each others mouths in the past) in the photo. Just make sure he is still able to eat, he might need some help with vegetables (you may need to cut them into matchsticks for him and maybe even hand feed them to him if he can't move his mouth very well).
On the bright side I have had similar wounds on some of my boars in the past, they typically heal very quickly.
Do you have any idea what caused the fight? Thing such as cage size, new introductions, incorrectly done introductions, one entrance hidys, age, a girl nearby, and illness can all be possible causes of two boars fighting. You may need to house them separately for now, at least until the wound heals. Did you catch them in the act of fighting or had they already stopped by the time you got there and saw the wound?