Just tell him to stop doing it. Otherwise you'll constantly be in a tit for tat battle, and if that's not what you want, it's a pain in the ass.
My next door neighbours son and daughter used to insist on beeping their horns as they were leaving the driveway as a way of saying goodbye!
When they left late at night / early morning it was annoying, but when I asked my neighbours to ask them to stop doing it (politely), they were more than happy to oblige, and could see my point.