From ebay: http://pages.ebay.com/buyerprotection/sellerFAQ/index.html
Is there a time limit for a buyer to open a case?
Generally, buyers have 45 days from payment to open a case—the same as with the current PayPal dispute resolution system. Exceptions may be made to buyers based on factors such as member account information, listing information, item location, and shipping method. You'll be asked to work things out with the buyer, but if you're unable to come to an agreement, we won't require reimbursement from you for cases filed after 45 days from payment.
What if the item is still in transit when the buyer opens an item not received case?
If tracking shows that the item was sent within the seller's handling time, but hasn't arrived yet, we may ask the buyer to wait. We may also extend the amount of time buyers should wait depending on circumstances like the use of a slower shipping service, or sending an item internationally.
That said, I would give him a few days to contact the post office and look into it. It would personally annoy (not that I can do anything about it) if he asks a question and then files a case right after. Give him till Tuesday or so to get back with you. If you arnt happy at that time then open a case. Just make sure you do it within 45 days of paying!!!!