When we're dreaming, our consciousness is on the astral plane, outside of our physical body, which is also where our deceased loved ones are. So I wouldn't get freaked out about it, I would be thankful for the opportunity to have met with him, you don't know when and if it will happen again. He wanted to send your son a message, and I am sure he is happy it got through.
That the "dead are conscious of nothing at all" is just false. My father spoke to me a few years ago through a psychic healer. It is also not true that "demons" go around impersonating dead people, because demons don't exist outside of the ones inside of us, aka our egos.