Do you know what happened? Is it a bite wound and was he bit by another animal, or could he have chewed through a wire and it's a burn injury? The important thing is to ensure there is no ongoing bleeding. If there is bleeding that won't stop or it is a deep wound, or the injury is due to an electrical burn from chewing through a wire, he really needs to be seen by a vet right away.
If it's a cut and the bleeding has stopped and it's not too deep, you may be ok and not have to rush to a vet but just get to one as soon as you can. Then you will also need to see if he is still eating, drinking, and pooping normally? Also is he behaving normally or is he acting lethargic, subdued, grinding teeth, or sitting hunched up in a corner not wanting to move?