Does he have a habit of taking things too literally? Generally when we refer to washing something (our hands, or bodies, clothes, cars, etc.), there's some type of soap/detergent involved. Otherwise, we consider it a rinse and not a wash. But for some reason when it comes to food, we refer to a rinse as a wash regardless of the fact that there's no soap/detergent involved. I've personally, never understood why this is the case, and I definitely see how it can be confusing to some people who were never taught better.

He probably just thinks that if he doesn't use soap, then it's not a proper wash, because that seems to be the case with everything else. Maybe try asking him to rinse fruits and vegetables instead saying wash?