Thank you for the subsequent posts, I'm still here.
It may not have come across from my posts but DSS feeling secure has been our priority from the start and DH has done his absolute best to make sure that happened.
When DH informed DSS of my pregnancy he was genuinely excited. I am and always have been on good terms with his mum and DSS feeling pushed out was something I worried about and did ask her about. DSS mum reassured me, several times, that he was ok with the pregnancy and excited to be a brother because he always wanted siblings. If DSS was struggling or felt confused/hurt then his mother would have said something, she's not somebody to beat around the bush.
Throughout the pregnancy DH had plenty of 1 on 1 time with DS, away from me, reaffirming the fact that he was and always will be a priority. DSS was genuinely excited and enjoyed being involved in the pregnancy / preparing for babies arrival.
DH still has plenty of 1 on 1 time with DSS now and they regularly go on day trips and do fun things without us, so he doesn't miss out on being able to do things for kids his own age because of the younger ones.
None of us adults involved believe this is as deep as unresolved feelings of abandonment. DSS is a happy and well adjusted child. If he gave any indication that he was feeling that way then it would be handled appropriately, we're not blind nor stupid.
This is simply a case of DSS not liking DS unfortunately. As others have said sometimes siblings just don't gel and get along.. and that's fine.
I don't think for one second he dislikes him because he's autistic as DSS just isn't that type of child, but DS' autism obviously adds another layer of complexities in that he can't talk or engage with DSS in a way that is appealing to him.
When I posted I wasn't looking for people to tell me that DSS needs to be punished or forced to play with DS, I was just looking for my feelings to be validated - the fact that I felt sad about DSS' feelings toward my little boy.