Maybe he needs an imaginary friend or something. I have one and I talk silently to him, so I no longer get a strong urge to tell people random stuff. It helps me work stuff out, too.
Also, you said he doesn't like the idea that he's upset someone? Does he realize he's done it right away and feel embarrassed? That's what motivated me to just listen and not talk unless I had to, because I realized I always felt embarrassed after I'd said something. So maybe you could try to get him to think of it that way.
That's the best I can come up with, anyway, for whatever it's worth. :B
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"You gotta keep making decisions, even if they're wrong decisions, you know. If you don't make decisions, you're stuffed."
- Joe Simpson