Am I spoiling son/being unreasonable ?

Hi all any advice would be appreciated.

  • i was a single parent until meeting my baby sons father 3 years ago
  • his elder brother is now 5 and has displayed asd behaviours increasingly since the age of 2
  • his paediatrician said she believes his autism is mild and diagnosis may take a while, also he may not be classified as severe enough to ever get help
  • my son shows huge levels of frustration, he can lose the plot over the smallest thing. He develops obsessions over certain toys which it seems no one else can touch.
  • my sister has an autistic child and advised to promote sharing with less important toys but explain to his siblings/step siblings that certain toys are just for him. She said that with a new toy or for instance dinosaurs with her son he can become very anxious and it almost causes fear of losing control if someone takes them or moves them.
  • my sons current obsession is Lego, he was bought some for Christmas and as usual at Christmas had a few meltdowns when things didn’t go to plan! 
  • I know this sounds trivial but his stepdad shouts at him whenever he has a meltdown. He said in front of him “what a stupid present to buy a kid like him” when he believed it was from Santa. 
  • He consistently says things to my son like “don’t start telling tales on **** just because your mums walked in” referring to my stepson who can be awful to my child but is very crafty about being caught. My partner believes I spoil him and I’m not assertive enough.
  • my son was building with his bricks and his stepbrother would just remove a piece of Lego and start laughing when my son had a meltdown. My partner shouted at my son for not sharing. 
  • He told him his new pyjamas were “pathetic “ and when my son told me my partner shouted at him for telling tales. I though he’d done well to not retaliate! I feel like I can’t protect him enough as my partner doesn’t seem to accept he has issues and categorises him as spoiled and naughty 
  • The difference being when my child has a meltdown or becomes aggressive it isn’t hidden, but he will punch my son, shove him etc and deny it in front of his father. My son is 5 he is nearly 7. 
  • My son told me one day his stepbrother had made him touch his willy and then did it to my son. He told his dad it was my sons idea and it was only as my daughter had overheard it we knew the facts. 
  • I love my son, it’s hard sometimes but I think after bringing up 4 kids alone we managed well. Now with my baby son and partner I dread spending time with my stepson there. If he isn’t in the equation there are no issues but my son obviously needs to learn some tolerance. I don’t know what to do!