I dunno Pruni. DD is quite alot like this and I'm not so sure how to handle it either. Pre-School say how quiet she is, although she gets on with things fine whilst there.
She's a totally different character at home. They wouldnt recognise her I am sure of it.
However, I think DD's hearing problems may be partially connected to some of the behaviours she has in 'public' settings.
i dont think trying to make him "change" will achieve anything, except to perhaps give him the lifelong impression that you dont like him as he is maybe??? Allowing him to be as he is/who he is, but trying to engage him more is the best advice I can come up with.