If it was a tick I doubt it would be this colour and you'd be able to see legs.
The only thing I can think of is an aural haematoma, but that's purely guesswork on my part as I've never seen one, I just know that dogs can get them.
It's very angry looking and all I can think of to suggest is to take him to the vet in the morning if it's still there and try to stop him from shaking his head in the meantime. Does your vet have an out of hours service that you can phone for advice?