Fight fire with fire. Figure out which topic he doesn’t care about, and start talking about it all the time. Like, literally all the time you see him.

Could be a new hobby like flying a kite or an obscure interest like the history green colors used in the textile industry. Ideally, you would pick something you can imagine yourself being passionate about. Look up some videos on how autistic people talk about their favorite topic, and you’ll get the idea.