Try and have fun here, be creative. Say if you had palmed his card and he takes out another one just take the deck back (really quick example) drop the palmed card on top of the deck (in secret) and create some misdirection. Ask if that is his card again and then quickly flip the top card of the deck over (his real card). Take the card he has and load it onto the deck. Now do any presentation patter you want like 'It is? for a second there I thought you were thinking of another card' and do a snap change. Or don't bother flipping over the card and do a shapeshifter change, or do an erdnase, go nuts. The only limit is your imagination my friend.
Great Idea. Thats what I'm gunna do if thats what someone says!