Do you know what he hit himself on?
Looks like a bruise to me, but I am NOT a vet.
If you know what happened, I'd not worry too much about it. If you didn't see anything happen and this just appeared, I'd be concerned (less so if he had the opportunity to do something to cause bruising that you just didn't witness)
Ryan had bruising like that when he decided to not use the stairs on the deck, but jump up and hit the edge before the rails were put on...
Those were taken right after he did it - they got a bit darker and spread a bit more by the next day.