Have you thought of meditation? I know it may sound out there but it can help. He may be more calm in his alone time because it is the one time that he is fully involved in an activity to the point where his mind stops all the other clutter for a moment. He may be one of those kids that sees, hears, feels, experiences so much more than the rest of us.
Imagine sitting in a classroom bored to death already and seeing the kid in the back picking his nose, the girl at the side sharpening her pencil, the kid next to him playing with his notebook, the teacher giving a lesson. Hearing the air conditioner going in the corner, the squeak of a pencil eraser ect. You get the point
And in the mist of all of that trying to entertain himself in his head. How could he not blurt out? Unless you've experienced this it's hard to explain. I know that most people in general don't experience things in this way. For some people it's that all at once all the time over stimulation.
The meditation provides a great peace in the noise and helps to train the mind/body to do what you want. No blurting
It's a long term solution, you won't likely see a difference in the first 6 months. When you start to see a difference it's amazing!
