I'm not that strict on this, but...
in reply to a message by I_Love_Jesus
I think it should be short for Thomas. Even if you don't like it, it gives him an adult option, and looks way better on job applications. Plus everyone will assume his full name is Thomas. I dated a Thomas. He hated Tommy, but liked Tom, and Thomas was acceptable -- something respectable. I love the name Tom, but Tommy I'm not crazy about.