it would have been polite for him to thank you at the time of asking. but if it was in email some people don't like to send another email just to say "thanks" as that seems superfluous. a lot of people try not to send too many emails as it's considered a bother. like deli said, he wouldn't know if you were contacted or not unless the potential employer mentioned it to him when he's hired. and chances are the employer wouldn't even mention it.