I’m going to say any or none of the suggestions here may be right.
And some of them, like Inner Light, are awful choices simply because their impact is very dependent on having the context the rest of the series and characters.
The main thing is that Star Trek has a wide variety of tones. The way to success is to provide excellent examples of very Trekie episodes that are in the genre or tone that your brother already likes.
Don’t show them action if they like cerebral mystery. Don’t show them romance if they like action. Don’t show them intense drama if they’re into comedy. If they’re into animated comedies or anime, start with Lower Decks or Prodigy not TNG.
Examples from this perspective…
If they like psychological horror, then TNG’s ‘Schisms’ or Voyager’s ‘The Thaw’ might be best.
If they like action, Discovery’s two part pilot might be the one or even the movie Star Trek (2009).