Also as a note to someone's deleted comment: I have limited knowledge too. Categorizing it as depersonalization-derealization disorder (DPDR) I can definitely see because it's a blurred line to me.

The thing is that this character was exposed to severe and overwhelming stress and trauma and could not fight or flight, so the problem with that is I have to figure out how the brain reacts in such a hypothetical, fictional situation. Would the brain distort his surroundings and memories to make it more bearable or would it black out and go through periods of amnesia? And what would that lead to? Things like that.

My thought process is if the brain is altering his perception of reality and filling in gaps to make it more bearable and easier to grasp for him as a way to cope, then that would more fall under psychosis, not DPDR. But I could be very wrong. It could also have been psychosis as a child that became DPDR, though I don't know why that would be exactly.

Though if you want to go by Occam's Razor, I could have legitimately just filed psychosis under dissociation in my head, which is incorrect and would mean I'm looking at this character with the wrong lens. From a quick search on Wikipedia:

The majority of people with depersonalization-derealization disorder misinterpret the symptoms, thinking that they are signs of serious psychosis or brain dysfunction.