Don't need to ask him which cards he likes.
So many cards. He won't have clear idea which ones he wants. He will just want to have more, more... more cards :)
I bought many Yu-Ji-Oh cards for my son. But till yesterday, he still wants more cards. That is only thing he wants as his Christmas gift.
I bought many cards from China (very cheaper. $10 for over 200 cards). But my boy doesn't like them that much because the cards are printed in Chinese.