Hi BobThe best way to handle this would be to create a new account for your Grandson and make the purchase in that account. You can then give him the account details after he receives the gift. This way he will have access to the account for downloading and should he ever purchase any additional content it will be registered to the same account as is required.
If you have already purchased CTA in your own account then you can contact Support using the Support Form below and they will help you transfer it to a different account.
http://www.reallusion.com/CustomerSupport/User/QForm.aspx
Hope this helps clear up any confusion.