who's the kid and the cartoonist? *spoilers*

I've seen the whole movie but i can't really figure out chang shik's relationship with the boy and the cartoonist.

At first i thought the boy was his son and the cartoonist his ex, but he says his ex was taken from him by a rich guy, which doesn't fit the picture here, and his grandmother for some reason mistakes him for his father i guess, and then yells at him for not taking care of the kid?! Does that make the kid his half brother or something? and the cartoonist his stepmother?