Do you have full tracking on the package, that will show delivered when he gets it? If so then just upload the tracking and hope for the best. Be sure to mention in the case that it takes a lot longer than 17 days for international delivery.
If you don't have full tracking (with delivery scan), then persuade the buyer and eBay that they need more time for delivery since it is international. Try to show them any proof you have that you shipped it to the right address, and the date shipped (i.e. receipt from post office, or screenshot of online shipment receipt/endicia etc.). In a case where there is no full tracking, and the buyer still says he never received it, you will probably lose the case. Bottom line is - tracking is your only proof of shipment/delivery. Always use it when it's cost-efficient.
Good luck!