Your kid could have some what of a point about the showers. It’s hardly like he is trying to impress the love of his life at 9 years old, if you don’t live in a very dirty hot city you can maybe go 3 days without a shower and still be fairly clean.
However, teeth should be brushed 3 times a day, once after ever meal. If you snak, you have to at least use some mouthwash after snaks.
I never used to brush my teeth as a kid, I can’t even begin to describe the pain if have felt some times as a result of this past decision. It is not nice when your wisdom teeth decide they are going to push thru and shatter an already half rotten tooth. Picture a 6ft 6in grown man cralled up in a ball on his bed, on the edge of being in tears as he misses his mother, and you start to get some idea of the pain we are talking about. I have even thought to my self “If only I could pull my jaw off or chew on some rocks to get the bad tooth out” in the past.
I have two of my back teeth missing, and three others are broken in half. I now wash my teeth three to four times a day, my teeth are nice and clean and they don’t hurt, but no amount of brushing will undo what is done.