I recently did this to create a new account after returning to the game. The first thing you have to do is log out of the GameCenter in the settings menu of the iPad itself, remove Star Wars Commander and than reinstall it. On the iPhone you need shut down Star Wars Commander and make sure that he is logged into GameCenter in the device settings menu. If he is not logged into GameCenter on the iPhone than do so and than start Star Wars Commander. If there is a Star Wars Commander game file already saved to GameCenter from when he played before he will be asked to choose which game file to keep, the old one that’s in the cloud or to replace that one with the new and current one that’s already on the iPhone. Choose to keep the new and current one. The bottom line is to make sure that GameCenter is tracking the progress of the current game on the iPhone. From here all you need to do is log into the GameCenter on the Ipad and than start the game after you reinstalled it. When he does he will be prompted to log into GameCenter and than the game file he just created using the iPhone will sync to the iPad.
It sounds complicated but its not, it just a lot of steps. In essence your telling the GameCenter cloud to track the progress of the game file that’s on the iPhone and than logging into Star Wars Commander on a new device and letting GameCenter sync up with the new device. I have done this multiple times with different games, its all done the same way. And if he ever wanted to replace his current game and start a new one from scratch than repeat the same process except this time after logging out of GameCenter and reinstalling Star Wars Commander start a new campaign before logging into GameCenter and than quit the game, log into GameCenter, start the game and tell it to replace the old file in the cloud with the new campaign on the device. The old switcharoo.