I read a great case study in a medical magazine about a child who used to shout all the time. The child psychologist made a cardboard badge with a second circle of cardboard on top with one of those little brass "butterfly" pins, so that the top circle could turn IYSWIM. She told him this was his volume control, and when his mum turned it down, he actually spoke quietly!!! It worked. I have no idea if it would work for you, but i guess it is worth a try.
On a more serious note, I would also get his hearing checked, because if he cannot hear himself, he will have no idea how to ajust the level of his voice. Good luck.