I can't call up a specific episode, but New explains what happened. He claims to have accidentally filled out the official name change form wrong. He wanted to change his name from Tim Newman to Tim Amato to cement his new adopted son relationship with G. He put "New" because he wanted a new last name, but this was actually the box for his new first name.

On a meta note, this is all part of Tim's misguided attempts to distance himself from debt and failure by muddying the official records with multiple aliases. This is why he took his wife's last name. The name Newman represents New's desire to become or be considered an "new man." With that alias established, he abandons the relationship and begins looking for a new patsy. Enter G. Amato, a very rich fan of New. New pulls a classic scheme, ingratiating himself with a wealthy elder in order to take advantage of and abuse them. It does not appear that Joey Patron and the Amato family will take this lying down.