If the case opened is still for an item not received you can give paypal the delivery confirmation number and they should take off the hold. I'm surprised that Paypal wasn't holding your money automatically for 21 days as they often do that with new sellers.
Has the buyer explained why he says that it is not his order? What does he want to do...has he suggested that you give him a partial refund? In situations like this I would probably tell the buyer to send the item back for a refund. If he's fibbing then its unlikely that he will send it back.
To get the buyers phone number, you can request their contact information from ebay.
http://www.ebay.com/sch/ebayadvsearch?_sofindtype=9