That might be an idea Fiona. One of the biggest traits for ASD people is to take things very literally, and because of this they have a very hard time with fantasy/imaginative play that isn't simply an adaption of real things. Evan, I'm guessing, would know what a bunny looks like, how big they grow, what they do - the easter bunny (a very magically creature) would break all those "rules" to be an exception to everything he knows. Santa wouldn't be as much of an issue because he is just a man who dresses in a certain way and travels at certain times of the year with the particular job of delivering toys, you could most probably get away with the tooth fairy too being more human-like and performing more human-like jobs. However, if he is very literal, if you do manage to convince him in any of these things being real, it will be a huge (and I mean HUGE) issue when you finally have to let him know that they are not because you are changing the goal posts (which never goes down too well).