Have you ever tried humor? I can get very defensive when I turn out to be wrong and end up sulking :-)

You said, "If someone stops over, he puts on an act and pretends that everything is all right. Once the comany leaves, he's back to his usual silent treatment." I would try to replace my frustration with good humor and try to gently tease him out of his moods. If he can't tease, can't get out of his moods either by himself in a reasonable amount of time or with help from you,

I would not cater to his moods though, react in any "upset"/frustrated manner. If he can't be in good humor when you're around, I'd go out. Being "concerned" and frustrated makes his behavior "work". If you can be light about it and tease him out of it, great but otherwise, I wouldn't put up with it. If he can't see it now with your help, I don't think it can get any better later?