I will agree it can be anything, at 6 most children have all sorts of friemds they imagine and it can be merely that. And believe me they are as real to the child as a physcial person standing there.
It can be a reoccuring dream or just a good imagination seeing shadows and other movements
Or if could be something that some call "ghosts" some people are more open to see things than others, I do believe children can see angels at times when we don't see them.
But all the questions asked so far are good ones. As would setting up a video camera in the room and see if there is a real interaction between him and "something" even if you can't see it, or does he just sleep though it and then wakes up thinking it happened.
And a good house blessing can never hurt either