You need to give more information about the situation. Did he and his 'wife' fight? Are they both spayed and neutered? Have he and his 'wife' been bonded?
It seems like a bite mark to me, although other members may have other ideas. Is it possible that the two rabbits fought and she bit him?
Keep and eye on the wound, make sure it doesn't get infected, if you are concerned, take him to the vet. You also need to make sure he is eating and pooping properly as he may go into stasis if not. Make sure he eats hay! That is one of the most important things you need to monitor, if he does not eat hay he can go into stasis.
Hope your bun will be okay <3