I think you'll be hard pressed to find someone progressive enough for lemmy to like that will also be interesting to a Joe Rogan fan. I would recommend Destiny though. I wouldn't say he's like Joe Rogan, but he frequently does debates with commentators of all politcal positions including those with similar beliefs to Joe. He has an aggressive, confrontational style that would be your best bet at trying to demonstrate the weaknesses of the views Joe would advocate for.
If you're not familiar with his views, Destiny could probably be described as a left-leaning liberal institutionalist moderate. His community can attract a variety of viewpoints and is relatively accepting of criticism for Destiny so your brother would have an easier time interacting with them than someone like Hasan.
It's nice you are trying to look out for your brother like this. Good luck!