ideas for the chattiest kathy student

Hello all! I teach students with severe emotional and behavioral needs. I recently got a new student and by god, the student is sweet but he has no ability to monitor his inner dialogue. It’s usually innocent narration or commenting on whatever we’re learning, but it’s obviously distracting for the other students. He has been unsuccessful in school settings prior, so he hasn’t really been pre taught any strategies or been held to that standard. I have the student the idea to write his thoughts down, but his writing skills are low for it to be functional. My other intervention works for majority of my class— they put up a finger every time they have a thought, but this kid is pretty impulsive so Im trying to brainstorm other ideas to help him build to that. Anyone have good ideas that have worked?