Name changed because its a bit identifying.
DH is the youngest of 3 siblings. He is possibly undiagnosed AS (certainly has a lot of traits). He says he believes he is. He was taken to CAMHS or its equivalent when he was a child but in the 80s, ASD was really poorly understood/diagnosed. He never got any support. He was definitely a handful and did give his parents a rough ride at times. The thing that he did that seems to bother him most now is petty theft (which is absolutely wrong-but I wouldn't say is massively unusual). He would be physically punished for this (which breaks my heart because he normalises this because "he was a little shit"-but what was done to him is not OK). His 2 older siblings were "golden children" and even as adults were treated preferentially to DH.
DH is a good person. He did make mistakes as a child (haven't we all!) but he is nothing like this now.
He has spent most of his adult life making amends for this and up until his parents died, had a good relationship with both. For example, he paid their mortgage for a time in full as he felt he owed them this (this ran into several thousands, the petty theft I refer to is a few quid for cigarettes/cigarettes themselves) I am not minimising what he did-just trying to give a bit of context to the scale of it.
I was close to both his parents-and as they got older, I would help them with cleaning, cooking etc and DH was treated a little, but not much, better.
His siblings on the other hand have not "turned out" well. One behaved appallingly after the parents passed away, particularly to DH. So much so he did not even get a chance to retain a picture or momento from his parents house. It was cleared out before the funeral. The other is serving a prison sentence for a very serious offence.
Despite this, DH is still troubled by how he was as a child. I have tried lots of times to comfort him, tell him that whilst obviously wrong, what he did isn't so unusual, he has more than made up for it, the physical chastisement was not OK-but he just won't accept this. This has got worse for him since his parents passed-and I don't think he will process the grief properly until he forgives himself.
How can I help him? We can't undo the past, after all.