The simplest way I can think of (besides paying in Canadian dollars on his Paypal account backed by a credit card and let their systems work it out, like a normal person would) is to look at his purchase on eBaydotCOM, where you will see both the USD value and the approximate CDN dollar value.
Then go to Paypal and send him a Send Money request for the US dollar amount.
And then add him to your Blocked Bidder List.
