Invisible man

Charlie has been going to nursery for about 18 months now.   Every time I pick him up I ask him about his day and I get the same response 'I just played'.  If I ask who with he says 'a little girl' or 'some children' or some equally vague answer, it is very rare that he will mention another child by name.  He also insists on calling all the staff 'Nursery' which I find quite embarrassing but they find hilarious!!  The staff have told me that there are a couple of children that he really gets on with and they always play together but he will never talk about them outside of nursery.

Recently the nursery decided they were going to have a party day, they had the normal party foods and games and also a magician.  When I came to pick Charlie up he was very animated and was chattering away about all the things they had done and, in particular, the balloon animals the magician had made.  As he was in a chatty mood I thought I would try broaching the subject of his friends;

Me:  Did your friends enjoy the party?
Charlie:  Yes they got balloon animals too
Me:  Cool, who got what?
Charlie:  They all got balloon animals?
Me (taking different tack):  Did you all play party games?
Charlie:  The other children did but not me
Me:  Why not?
Charlie:   They didn't ask me
Me:  I am sure they did
Charlie:  Nope
Me:  Why not?
Charlie:  They couldn't see me
Me:  Why, were you hiding?
Charlie:  No (whispers conspiratorially) they couldn't see me because I was invisible because they just don't believe in me

Ummm that was not the answer I was expecting!

As a footnote I should add that I did check with the nursery staff the next time he was in and he has been joining in fine with all the games and at no point did he turn invisible..