You go here to cancel his bid. At the same time, it is probably a good idea to Block him.
http://offer.ebay.ca/ws/eBayISAPI.dll?CancelBidShow
However you could also show him how to cancel his own bids. Not only is this less work for you, his own cancels show on his feedback page and are monitored by eBay. Too many cancellations and he will be restricted.
http://offer.ebay.ca/ws/eBayISAPI.dll?RetractBidShow