It looks really grim although not very deep and he doesn't seem to be in pain. My neighbour (bless him) is going to take him to the vets for me because I've got SPD so I can't manage the walk.
I can't figure out how it happened, at first I thought he'd been in a fight again but it looks like a graze. My neighbour thinks he might have been hit by a car.
I might start keeping the cats in at night but I'm not sure about it. They used to be quite outdoorsy but there are 3 new cats in the neighbourhood and they are all nasty buggers :( so my two don't go out much any more.