I just got a copy of the new Washington History journal as I've got an article in it and found this. He was a Post cartoonist back in the day.
Slovick, Lyle. 2007.
George Y. Coffin: A Schoolboy's Life in 19th-Century Washington
Washington History 18 (1&2): 98-119
There's something online about him too - perhaps from George Washington University. I'll try to poke around and find it unless someone beats me to it.