I recently received a buyer, who left a negative feedback for the product he purchased, and claim the the product doesn't work.

I ordered a full refund and allow him to keep the product so he doesn't have to return it. Then I sent a feedback review to him. It seem like he doesn't want to change. what options do I have? I just started selling on ebay, one negative feedback means a lot. I believe that I have provided the best service that could possible be given by a seller. 

Are there any method I can fix it?

So far after I did some research, only found "asking buyer for a feedback review" and "SquareTrade mediator"(this seems like has no longer providing the service), 

Please guys, I need help from you all.