He hasn't- yet. ;-)
If he wants to, he can, the registration form can be printed out at
http://www.challengertaregistry.com/register.html
Years ago, Tom Quadrini saved some of the old SIAC club membership forms from going in the trash. He let me borrow them to make copies. There are three forms with your dad's name & the T/A VIN on them. Email me with your mailing address & I will get them to you so you can surprise him if you like?
Barry