How do I help DS with this? Really confused

[Odearr]

Ds is 11, has asd and adhd incase that is relevant.
He's been on and off upset about this all week, he's just been absolutely sobbing for the past hour over it.
But he's been getting so upset about something that he says he did years and years ago, when he was about 5 years old, I don't even remember this ever happening tbh but he says he got really mad in the middle of the night and ripped pages out of his flag book and he said he feels really ungrateful and wishes he could go back in time to when he was 5 and undo it and that he doesn't understand why he would have done that.
I've told him that's it's no big deal and it's just something that happened when he was little, we all do silly things when we were small etc but nothing helps he's been on and off upset about this all week.
I have ocd myself and I'm so worried this could be something similar to that or if there is some other reason he could be so stuck on this?

Pay for private counseling. He needs coping mechanisms now to deal with this and future flare ups. I would take this very seriously.

This does sound like some kind of OCD. I am no expert but my DD1 is high up in MH and was living with someone with OCD so we all learnt quite a lot about it.
Separately my DS2 has mild autistic traits and add this is something he would have obsessed over at that age. I think being an 11 year old boy can be really hard, especially if they are quite clever and over thing things(anxiety). Same son when through high school wishing he was not clever and in his words" not worry just accept things."
I still spend a lot of time discussing his feelings and reassuring him, even though he is grown up. He has lots of friends however he is " a lad" and doesn't talk to friends about important things.
I hope your son finds some peace soon.

I have autism and I get like this about things, it’s definitely a sort of OCD / hyperfocus thing. I know this might sound like a really daft suggestion but could you buy a couple of flag books and then encourage him to rip them up? It might break the intrusive cycle by him realising it isn’t actually a big deal, sort of laugh about it with him. I’ve done similar things with my own Ds (who also has autism) and it helps to break the rumination over things to repeat them in a different way.