Imagination

this could be a early symptom of something else.

for example, you could identify this belief in a imaginative thing to be a delusion, then see what strong belief in delusions is as a symptom and what its linked to.

on the plus side his creativity should be valued and he could likely become a good author if he then writes down his imaginative ideas and compiles it all 

My eldest had two imaginary friends, Dreamer and Deeda, Also an 'elder sister', who, "Died in a volcano". It is a stage some children go through, I would not be too worried about it. Also, do not feel guilty about pointing out realities, it is unlikely to have any adverse effect, and reality has to be faced by every child eventually. 

You could try explaining what the imagination is, and then say that it was an interesting story and ask him to draw a picture of the planet, and then you could put it in a book for him. and say that imagination meant he was able to draw such a great picture. and that was positive. 

Kids have the greatest imagination, everything is fantasy and play, it can be extremely real to them. I suggest keeping an eye on him, he should and probably will grow out of this but if he doesn't then consider a child psychologist to help him work through this. I'm sure it's just imaginary play though try not to worry.

Sometimes, I feel like we're the humans; and the outside world that's Alien.

I can understand your anxiety but I think this is normal for most children autistic or not. Kids are very imaginative and when young imagine the impossible and it can be so real to them. When I was a kid I pretended I was a witch and did this for a long time, making potions and spells and talking in my own language. It worried my mum but my dad wasn't especially worried. He said I would grow out of it and I did. I expect your son will do the same in time.

I agree with the comments above, and will add that I don't think this is unusual for any children, autistic or allistic. I think lots of children imagine themselves to be aliens or lost princes/princesses or whatever. I guess it helps them adjust to the idea of having a separate identity from their parents. I very much doubt that it could turn into anything dangerous.

Hi, if I had a pound for every person on this forum who has said, “ I feel like an alien in this world”, I would be a rich man. It’s very much how we perceive the world, so much of it isn’t recognised as normal or suitable, while at the same time everyone else seems to find it all normal. In the end we just believe that we don’t belong here, I often told my parents, “ my real parents will come for me one day”.

I watched two documentaries recently.

One is about a young boy who believes he jumped and died from the twin towers when they were attacked.

Another is about an Egyptian scholar, Dorothy Eady, who believed she was once an Egyptian Priestess that had killed herself after becoming pregnant after an illicit affair with the pharaoh.

I imagine their talking about it helps them separate their two worlds. Be them born from imagination or, as they believe, past lives. Both of them have stories to tell! The latter used her belief to fuel an amazing career.

When I was quite young I said more or less the same thing to my parents. And I can tell you for a fact that I didn’t really believe it was true at the time but I kind of wished it was true. This planet is rather tiring it does feel very alien to lots of autistic people. we feel like aliens stranded in an alien world. But we don’t look alien and Our parents expect us to be normal; oh there is this crushing expectation of normality forced upon us through lots of tiny things most people don’t even think about.

but aliens don’t have to be normal they don’t have to abide by conventional rules or fit in neat pigeonholes. I don’t think he really thinks he is an alien I just think he wishes you'd treat him like one and thats something you might like to consider.

when difficult or contentious situations come up maybe you should ask yourself how would you treat him if he really were an alien. If you had disappeared in a beam of light one night and then turned up the next day pregnant with him, how might you be treating him differently today.