Worried about my youngest child after eldest just being diagnosed

I agree with Endymion in that it's quite possible he's picking up patterns of interaction from your daughter, my little brother was much the same. He's not autistic, and especially when we were young it was very obvious. He was very interested in playing with other children, very into team sports, lots of eye contact, no sensory issues.

As a quick comparison exercise:

* He would play in groups at playgroup pretending to be this, that and the other, changing the game with the input of other children coming in to join. I would sit and read to myself or sort boxes of toys (and woe betide anyone who came over and messed them up- i.e. tried to join in). 

* He thrived on football matches, playgroups and family gatherings. I hated loud noises and crowds and would hide under tables at parties.

* His language development, I'm not certain about- I'd have to ask mum. I know she thought I was "odd" with it though and she's never said he was- I was markedly echolalic (would repeat words and phrases and, later, whole paragraphs from books by rote), was singing before I was talking (classic "can physically talk, won't converse") and a very precocious reader (I was fluently reading the usual childrens' books at 2, the Lord of the Rings at 6).

* He watched a variety of things on TV and ate a variety of food. I watched the same VHS tapes over and over until I had memorised them and was just the fussiest eater- I went through a several-month phase as a very young child where the only solid food I would eat was satsumas and had to have any sauce, curry, etc. with a chunky texture blended until I was in my teens (with hindsight, blatant sensory problems).

On the other hand, he never quite 'fit in', especially as he got a bit older and things got more socially complicated for both of us. He would display some social behaviours you'd probably suspect to be autistic if you'd just met him. He still does and is very socially anxious on top of that.

However, it was clear to us (having the whole picture) that it was very much behaviour he'd learned in order to interact with me on my wavelength. Now he was trying it at school with NT kids and it wasn't working and (me being undiagnosed at the time) he had no idea why so that was /confusing/ and he struggled, hence the anxiety. 

He was having to alternate between one pattern of social behaviour at school and one at home. His instincts said "NT communication", but in the same way I'd learned to pretend to be NT to cope among other children, he'd been subconsciously teaching himself autistic-style interaction to use with me for his whole life! (After I was diagnosed, Mum called it 'aspie by osmosis'.)

I'm not sure what to suggest in order to help your son other than getting him interacting with plenty of other children as well as his sister. If he is neurotypical, at least your daughter has her diagnosis and knowing why his sister's different will stop him becoming as confused and frustrated by having to interact one way at home and one way outside as my sibling did. I think a lot of my brother's problems stemmed from not knowing why "X works with Emma but is absolutely rejected (or even ridiculed) by other children, Y works really well with my friends at school but makes Emma go into meltdowns which by the way are SCARY".

You should be able to explain that to your son if you need to, so you have a massive advantage.

If he's autistic, which is also quite possible- autism is highly heritable like that- then that's fine and I strongly suspect that, either way, it will become clearer soon enough as he develops a bit and starts interacting with more people outside his immediate family. :) 

I agree with Endymion in that it's quite possible he's picking up patterns of interaction from your daughter, my little brother was much the same. He's not autistic, and especially when we were young it was very obvious. He was very interested in playing with other children, very into team sports, lots of eye contact, no sensory issues.

As a quick comparison exercise:

* He would play in groups at playgroup pretending to be this, that and the other, changing the game with the input of other children coming in to join. I would sit and read to myself or sort boxes of toys (and woe betide anyone who came over and messed them up- i.e. tried to join in). 

* He thrived on football matches, playgroups and family gatherings. I hated loud noises and crowds and would hide under tables at parties.

* His language development, I'm not certain about- I'd have to ask mum. I know she thought I was "odd" with it though and she's never said he was- I was markedly echolalic (would repeat words and phrases and, later, whole paragraphs from books by rote), was singing before I was talking (classic "can physically talk, won't converse") and a very precocious reader (I was fluently reading the usual childrens' books at 2, the Lord of the Rings at 6).

* He watched a variety of things on TV and ate a variety of food. I watched the same VHS tapes over and over until I had memorised them and was just the fussiest eater- I went through a several-month phase as a very young child where the only solid food I would eat was satsumas and had to have any sauce, curry, etc. with a chunky texture blended until I was in my teens (with hindsight, blatant sensory problems).

On the other hand, he never quite 'fit in', especially as he got a bit older and things got more socially complicated for both of us. He would display some social behaviours you'd probably suspect to be autistic if you'd just met him. He still does and is very socially anxious on top of that.

However, it was clear to us (having the whole picture) that it was very much behaviour he'd learned in order to interact with me on my wavelength. Now he was trying it at school with NT kids and it wasn't working and (me being undiagnosed at the time) he had no idea why so that was /confusing/ and he struggled, hence the anxiety. 

He was having to alternate between one pattern of social behaviour at school and one at home. His instincts said "NT communication", but in the same way I'd learned to pretend to be NT to cope among other children, he'd been subconsciously teaching himself autistic-style interaction to use with me for his whole life! (After I was diagnosed, Mum called it 'aspie by osmosis'.)

I'm not sure what to suggest in order to help your son other than getting him interacting with plenty of other children as well as his sister. If he is neurotypical, at least your daughter has her diagnosis and knowing why his sister's different will stop him becoming as confused and frustrated by having to interact one way at home and one way outside as my sibling did. I think a lot of my brother's problems stemmed from not knowing why "X works with Emma but is absolutely rejected (or even ridiculed) by other children, Y works really well with my friends at school but makes Emma go into meltdowns which by the way are SCARY".

You should be able to explain that to your son if you need to, so you have a massive advantage.

If he's autistic, which is also quite possible- autism is highly heritable like that- then that's fine and I strongly suspect that, either way, it will become clearer soon enough as he develops a bit and starts interacting with more people outside his immediate family. :)