I've heard of this happen to some children even I have known (relatives they couldn't possibly know about, visitations, conversations, games, etc).

It can really only go on at a young age...

Ages between 2-11. The beginnings and ends of the scale being the weakest times.

Keep this to yourselves and don't tell anyone else.

Let this child experience what he can while he's able to physically...

Childhood is a precious time. Don't ask him too many questions, don't feed into it too much, and keep your emotions to a minimum around him, just be positive and happy... and let him have his time.