Are you getting emails from his server telling you your messages have not been delivered? If not, the problems his, not yours. You've done everything you can.
Try sending an email with a return receipt. he may have it disabled but if he hasn't you'll find out whether he's getting them or not.
Finally if you message him through eBay's "contact member" he must get it, surely, or even eBay can't get him.
GG.