Send him a message through Messages. Then he goes here:
http://resolutioncentre.ebay.ca/
and clicks on We want to cancel the transaction.
You can't do this, he has to.
Then when you get the message, and he has refunded if you sent money already, you agree and leave appropriate feedback and Detailed Seller Ratings.
Do NOT agree until you have your money back, but otherwise this a normal and friendly way to end the transaction.