The solution: Something something, be the change you want to see in the world.

Personally, I think it doesn't matter if he joins the fediverse now or in 10 years or ever. It doesn't matter if the fediverse grows, dies or stays like this. He has the choice, now, to be part of a better internet, regardless of whether it will completely change the internet landscape as we know it or not. There's always going to be moments where a decision has no material benefit over the other and can sometimes even be more inconvenient. The only reason is: "That's what I believe in". What happens in the future is out of his control. He can simply pick his side.

This way, regardless of whether the culture on these closed-off platforms forever stays on there, he knows that his contributions to culture, may that be an upvote, a comment (yay!), a podcast or a post, is a contribution to all and therefore truly becomes part of culture.