Child with ASD and divorce (sorry, long)

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Hi there - I tend to lurk rather than post, but hoped someone here would have some good advice! In short, my soon-to-be-ex-(D)H and I are getting a divorce. Not my choice - DH has met someone else - but I have tried what I could to stay together, with no success and the split is now definitive. DH moved out around 3 months ago, shortly after breaking the news of his affair. He is around practically every day, and spends a lot of time with DS. I really wasn't happy about the manner of his moving out, though, because he just went one evening, without telling me he was not coming back. It took me almost two weeks to get DH to sit down and agree about what we were going to tell DS. Which was the usual platitudes about mummy and daddy loving him very much, but not getting on with each other, so we have to live in different houses.
DS is almost 6 and has a dx of ASD. He is in Year 1 mainstream. His language is coming along quite well, has even managed to make a few friends at school, is generally a little ray of sunshine.
Or at least was - for the first time in his life, he is being aggressive. There were incidents last term. In the last week at school, every day there was an incident of him pinching, scratching or otherwise attacking other children. When I ask him why he did that, he says "because I am not very good". He says he has lost his father - I tell him that's not true and that his dad loves him very much. Also, his best friend from school has had to leave and moved elsewhere - DS told me "ABC left because he doesn't love me".
What should I do? The school and I are enforcing the usual punishments for bad behaviour (taking away toys, red spot, etc.) and rewards if he behaves well. His school teacher thinks this may have to do with his autism, and the fact that emotionally he is at a 3 1/2 years old stage, 'the defiant stage'. But obviously, I think this is about DS feeling abandoned. I don't know what to do - I tell him that his dad loves him, they do do things together, we have put together a new routine and communicate it to him on his calendar. I tell him that I love him and I'll always be there - once he replied "But you're mum, you're not dad".
I can't make his father come back; God knows I've tried. What should I do? DS has never been aggressive, I am so afraid that the other kids will start not wanting to play with him any more

I think you're absolutely right and this is about your H leaving. Any changes can cause children with ASD huge amounts of stress and having his Dad leave is a huge change and incredibly unsettling, especially as it was so sudden.

I expect by your DS's logic, if Dad has gone so quickly like this then anyone can go.

It may help to discuss this with him in a factual way? I know my DS struggles when I put it in terms of feelings. I will see if I can find any books that may help.

Oh, I'm so sorry to hear this. My STBEX dickhead left in Sept following an affair and is with the OW and my DS2 12 with HF ASD has actually been fine about it. But all children are different and those with ASD seem to be more different than most. Wish I had more advice for you. My DS has no empathy at all but is generally a cheery chap. He told me off for crying a couple of weeks after dickhead left, because "that was 2 weeks ago." In other words, pull yourself together, woman! At 6 it may have been harder, I suppose, as he would have understood it less.

My only advice would be to make contact as regular as possible so it becomes the new routine and little and often rather than for a long time more occasionally. ie twice a week for a few hours rather than every other weekend. I say this because he's quite young and 2 weeks would be a long time between visits.