Now, i think you're worrying too much, granted "sexist themes" are played often in anime, but if it bothers you too much, it'd be better to have your child watch shows from another media i.e. cartoons meant for kids, etc.

Stuff like Ben 10, Steven Universe, Generator Rex, Arthur are great picks with none of the things you've mentioned, i've also found cartoons from the UK to be a lot more child friendly in nature too, stuff like "Lets go on a bear hunt" etc, come to mind.

Understand this, i am a 22 yo adult leading a normal/ happy life, i grew up watching some stupid low budget horror flicks cause my mom loved them, i also saw anime at an impressionable age of 16, so did my friends in HS, we had pretty healthy school lives. What i'm saying is that if you're concerned about a genre meant for young boys growing up in Japan i.e. battle shounen, and boys that age find the things you've just mentioned funny (ages 13-19), but that hasn't affected me in any way, i enjoyed and still enjoy watching crappy horror flicks with my mom and sis, and i still enjoy One Piece/ Naruto.

Also your son being 12, is way too young to be introduced to anime, especially battle shounen, there's other genre, but i'd assume he'd be pretty bored of the tamer shows, so as i mentioned shows like Ben 10 and Avatar (although not anime) would be better suited.