where are the clickthroughs going to?
they should be going to a pass through page on his site where a cookie is set and that logs the hits before being redirected to the target page. (this cookie should tell him it's a referer from you)
alternatively serve his ad in an iframe that is his and that way he can set the cookie in the iframe, of course he'd then be able to get all your traffic stats.
i don't use google analytics at all on any site, it is easy to track how a purchaser hit the site... perhaps you just need to teach him some basic analytics stuff.