I really refer to the most competitive. If he can’t get calc (and is thinking stem) what about bio, physics or chem? I’m surprised the teacher didn’t agree.
Tippy tops can review with the major in mind. It doesn’t matter if there’s, eg, a college of engineering you apply to. When the competition is fierce, it helps to have your ducks lined up. Adcoms can look for the prep (courses and activities) behind the major(s) you do state. In many cases, even Undecided needs some pattern for them to see.
No, you don’t have to take, eg, AP psych or sociology, for a social sci major. But it’s good if some activities point to the interest in peoples, cultures, behaviors, etc. And as many social sciences need analytical skills, why not an English or history teacher? IR does need foreign language skills, but that’s usually seen in the language classes taken.