That, and he’s not saying “Will, you need to go do good math.” He’s saying that if he’s still here in 20(?) years, he’ll fucking kill him. And that’s because he recognizes Will, thanks to his extreme intellect (which we see isn’t nearly limited to math), can go wherever he wants and do basically whatever he wants. He recognizes that unlike himself, Will chooses to stay there because he’s living in fear of the unknown – that people will hurt him, something he doesn’t have to worry about with his three Southie friends.

He’s talking about emotional growth and self-actualization for Will, recognizing that staying stuck in the place where he grew up and was brutally abused is keeping him trapped, because that’s where he feels “safe”.

I know this is a joke, but it’s completely misrepresenting the scene to get there, and it doesn’t even seem like obviously ironic misrepresentation.

Edit: IIRC, he doesn’t even say that he needs to leave Boston, just that Will still hanging around with them would be the problem.