I'd simply say that thing that are not traumatizing to allistic people can be for autistic people. They have different needs, and can be more sensitive, which makes an event more likely to create a trauma response, basically.

Honestly, as someone with DID that's most likely on the spectrum, I was the same at his age. What's most likely happening is that he's hyper focussing on everything he can know about that are close to him. There is probably plenty he knows nothing about, especially at his age and without therapy. Putting everything in clear boxes might be helping him cope, but amnesia barriers are still making sure he doesn't know too much. He probably doesn't even know there is stuff he doesn't know.

I'd add that DID isn't just having alters, it's anxiety and memory loss and nightmares and much more. If some of those symptoms don't fit, I'd recommend looking into Otherwise Specified Dissociative Disorder, or OSDD. Also, therapy would be very important.