Is it that he doesn't understand concurrent code, or is it that his knowledge of it is different from yours? If he's my age or even 5-10 years younger he didn't cover much with asynchronous web apps in school (I graduated just before Berners-Lee announced), but he may well be aware of concurrent processing in other contexts.
First off, anyone who did any GUI development even in completely-unthreaded 16-bit VB3 should be able to understand the basic concepts. DoEvents anyone? Throw a timer control on a form AND handle GUI interaction? Congratulations, concurrency if only at a minor level. Hell, anyone who's written GUI apps that trigger long database queries without hanging the app has dealt with concurrency.
Second, anyone who's done much with *nix command-line processing uses concurrency even if they don't know it. Piping anything through multiple stages? Each of those processes is running concurrently, either waiting for input or processing input possibly while the preceding stage is still chugging along providing more.
Finally, has he done any multi-threaded apps (or passed on that approach for scaling reasons)? At this stage I'd expect him to know what you're talking about, but there's still a noticeable difference between multiple threads and thinking the same way about web apps.