1 Novel 1 (found)

I read this novel that was about a guy who had a piece of paper that could hold stuff in it. He then created his own world in it, starting off with putting ants in that later evolved. Then humans and other creatures were in there and he chose one kid to help the humans but the kid thought the main character was immortal or something and asked to become like that. That's basically all I remember, so if you know the title, please tell me.