Not to be negative, but if your son isn't willing to discuss washing hands or using hand sanitizer, then he's probably not taking proper precautions. That means you should treat your house like it's in a public place by washing your hands, using hand sanitizer, and not touching your face unless you're doing so with a tissue. Assume everything is infected. (I'm not saying your whole house is infected, but I'm just saying practice the assumption so that you don't get caught off guard.)

Anyway, as you probably know already, younger people tend to get coronavirus very mildly and don't experience really bad symptoms, so they really don't care if they get it or not. (In fact, there are lots of news articles about college students taking spring break in Florida in hordes of literally 50-200 people.) They don't see the big deal. But they really need to be taught the importance of being safe because they are seriously risking the health of people who are 60+ or who have compromised immune systems (e.g., due to a medication, a health condition, whatever). Ask him how he would feel if one of his grandparents got coronavirus (if they're still alive) or if someone in your family with preexisting conditions like diabetes got coronavirus. I bet you he would change his tone.