If the friend isn't keeping in touch, can you really still call him friend? Friends do sometimes drift a bit, but if a buddy asked me for a response to a question, especially if he was trying to give me something desirable, I'd respond.
Offer a deadline, one that feels appropriate to the situation, and tell him he needs to claim his cards by then.
If you're feeling particularly generous and don't want things ending badly in any way, put his cards on eBay at the ultimatum date and sell them, then PayPal him the cash. It's hard for him to legitimately moan about the loss that way. Otherwise, just keep the cards and do what you wish with them. You were the contest winner, you're within your rights to do so.