you can play on the same account at the same time on different data centers, but you could not play together.

you could also purchase crysta for your nephew if he gave you his login info.

You can however have the same SE account but separate service accounts, allowing you to pay for both. however, i am unsure as to how this would work with the authentication token and if he ever wanted to separate his account from yours, he would be unable to.

Just make it two accounts and either buy crysta, buy game time cards, or use recurring billing on your CC as you see fit to trust.