Here's a good place to look to ID him/her:
http://www.saltcorner.com/sections/zoo/boxfishes.htm It sounds like it maay be the "Spotted / Yellow / Cube Boxfish", so you might want to look at that one first.
Fish that get to 12" adult size and up generally don't do well long term in a tank that's only 12" wide. Just so you know, if you really want to keep him, but can't afford the tank, there's always the option f going with a (indoor) pond.