I asked for advice for the very same issue here. The thing that worked for me was telling him he can keep some of those thoughts to himself. Repeatedly. It took a week or so, but it did work. I know the reason he was doing it was because his mom was narrating everything for him, hoping it would improve his literacy. Maybe it worked, he did seem advanced.
Pssssst...it works on adults too. "Hey! Lady on the Plane Who Hasn't Stopped Talking Since She Boarded! You know, you can keep some of those thoughts to yourself!"
Good luck. I know it can drive you batty.

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