I would assign him some tasks and sit next to him (or screenshare if you are no longer going into an office) while watching him as he works through what he has to do, and see what he's clicking on. If he can't repeat the mistake, then hopefully he won't do it again! Reinforcing the correct behavior may work out just as well as trying to figure out what the heck he clicked on! I've had to do this a few times with people who could not correctly share a Proof, yet every time I watched, they did it correctly. The incorrect sharing stopped after that.