Of course they could (see the thread about Tesco being shown up by a 7yo girl.)
It would be nice to think that in school, they'll never have occasion to do so, but... It was at school where I first realised I was a feminist. (Or possibly I just realised I was going to speak up because someone was treating me differently just because I was a girl, and not for any good reason.)
Sadly, he will probably have ample opportunity to point out things which are sexist, if he wants to.
At primary school in the 1970s my mum made a big issue of the fact that boys were required to stack chairs after dinner whereas girls were excused. She visited the school more than once to discuss it, and I found it hugely embarrassing. The school changed the system so that everyone stacked the chairs. I was still a bit embarrassed, but also proud and understood why she'd taken a stand on it.
She always challenged sexism, and explained to us what feminism was and why it was important.
By 10 or 11 I would happily identify and challenge sexism at school or in social settings. That included sexist remarks by teachers and other adults on many occasions. It was something I felt I had (and have) to do, part of being me.
Oddly enough despite having the same upbringing my younger brother was far less likely to be vocal about sexism, though he would consider himself a quiet supporter of feminism.