It sounds like what he is doing is "dumbing down" his oral language when speaking to people perceived as being in an evaluative position, so that he doesn't exceed his self-conscious limits. Sacrificing complexity for accuracy, in other words. Since these other listeners don't have the broader context of his high verbal intelligence, they don't realize that he is actually restricting his range in order not to embarrass himself, since his restricted range is grossly age-appropriate. He probably has no idea that this is what he is doing; I see this frequently in written language with vocabulary and poor spellers (they use only words they can spell confidently, often multiple standard deviations below their oral vocabulary).
And I am always delighted to help.
