As someone who has sold - I understand his frustration but you are right - the ebay rules say it is his responsibility to ensure delivery - if he has proof of postage he can claim back from the PO after 15 days but he cant make you do this and if it does not arrive ebay and paypal will find in your favour - so I would politely say that you fully accept that he posted it but unfortunately it has not been received.
Again as a seller this is very frustrating as he has obviously sent it. However he has to open dispute with rm not you. Many sellers say they will forward proof of postage to you should item get lost but as surrey says ebay/paypal would find in your favour. Maybe email him ask for refund and explain that if it arrives you will contact him but he has to open dispute and hopefully this can be sorted amicably without bad feedback etc!