Diagnosis

Hi all. 

Stepson has recently been diagnosed.  His father isnt taking it well.  He doesnt believe that his son has ASD for many reasons the main one being that his "traits" are only evident when he is with with this mother - not in school or when he is with us and his step siblings.  He is a very happy bright sociable child who loves life. 

He does however have an older sibling with and ASD diagnosis. His older sibling doesnt like him and really isnt very nice to him at all.  Is there any chance that he could be "switching" on and off?  I don't even know what i mean by that but the behaviours that his mum reported where exactly the opposite of what we see on a weekly basis.

 Is a misdiagnoses possible?  One of the things that they said was that he wouldnt elaborate on questions - tbh he was crying and didnt want to be in a room with strangers so one word answers where all that he was prepared to give.  Also he pronounced tomato the americian way so apparently he speaks with a non regional accent.  He watches youtube a lot when he is with his mum.  

Both his older siblings have an ASD diagnoses and up until the last few years outside of school they were the only children that he socialised with and tbh he copies a lot of stuff.  Like he can be very happily eating his dinner and talking away to everyone at the table and his older brother might refuse to eat something because it doesnt taste right etc which is fair enough and the next minute he says exactly the same thing that his brother has just said and refuses to eat any more of the food that he was cheerfully eating two seconds ago and stops talking? 

I haven't a clue about any of this