12 year old struggling to accept parents split years later

Hi, could I ask for some advice please?

My 12 year old (AuDHD) is struggling to accept that his dad and I split. It was around 4 and a half years ago. His dad and I are still friends and I have become best friends with his new wife. They have a little one together now. We co-parent together and want to support him as best we can.

Lately our son has been having attitude problems. I know, he's a preteen, it's to be expected. I remember I was absolutely awful as a teen. He's being disrespectful towards us all but he's being the worst with his step mum. She's trying her best and is so supportive and caring towards him but his attitude and disrespect is really bad with her especially. We have told him so many times that it is not acceptable and we are punishing him for it. But he has recently told me that he wants me and his dad to get back together.

As someone who has been through parents divorcing as a teen I get it. I spent years wishing my parents would get back together. Then I found out the truth of why they split when I was around 17 and I accepted that no, it's not happening. (I have AuDHD too by the way).

Our son also struggles with change and over the years we have had some hard times, bouncing from one home to another, there was a point where we ended up becoming homeless (we were put in a rough B&B for a few months when he was around 3 or 4). Throughout it all the one constant for him was me and his dad. So he kind of clung to that. My mum has mentioned to me a few times before the split how she had noticed he clings to the two of us and see us as his rock. So when we split he has struggled to accept it.

We have explained that we are never getting back together, even if his step mum was not here we still wouldn't get back together. That we were not happy together and that we are really happy now in our own homes and his dad is happy with his wife and their little one and I am happy being single (I have had a relationship since the split for a year but I prefer being single).

His dad said it's been over 4 years and he should have accepted it by now but as someone who has been in a similar situation I can understand his feelings. Obviously this is really hard for his step mum and unfair on her, she's genuinely so sweet and wants the best for our son.

So I am asking has anyone been in a similar situation? Could anyone give any advice please? How can we best support him through this? How can we get him to stop the rudeness towards his step mum? We really would like him to understand and accept it is fully over but that we are all one big supportive unit around him. Thank you in advance.