What do you do if you ask him to find their card in your deck and they pick out another one delibratly to ruin your trick?

If other people have seen the card, that just makes him look like a fool. If that's notthe case and he picks an indifferent card, ou have two options at this point.
1. Tell him that you can read his mind and that card is not his. Then find his real card and blow him away.
2. Explain to him that if he doesn't want to be entertained, then you'll find someone that does. Then walk away.

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!