I personally love DS9 and think that it has aged better than TNG. I think its S1 is not amazing, but certainly one of the better first seasons. It has plenty of good enough episodes that don’t depend heavily on the upcoming plot and leave your brother in a good place if he wants to start watching for himself.

A few suggestions would be:

• In The Hands of the Prophets: Overall an almost prophetic episode, in the most terrifying way possible. A well-done drama episode with great political commentary. A lot of the season built up to it, but it’s such an early period in the show that it’s not TOO much context
• Dax: A full helping of everything Trek, from alien trials to crew collaboration to space mysteries. It basically explains the Dax thing for you, just leaving an interesting story. Vortex: Odo-focused, but also has some “crew on space mystery” bits. Also makes a mystery of core information in the show, meaning minimal canon dependency and once again leaving your brother in a good place to watch.
• Duet: Strong Kira episode that’s also a good summary of the Bajoran-Cardassian conflict.
• Captive Pursuit: I think it’s a solid, typical Trek episode. I think the only impression issues it might give are it’s very O’Brien-centric, and it might register a bit on the “aliens who represent no particular real life ethnicity but are still kind of iffy”-o-meter. But otherwise, it’s a low-canon, medium-quality episode.