I've actually been thinking about this recently, trying to figure out how to put this to the team when I go for diagnosis - currently I'm undiagnosed but strong suspicion of asperger's both personally and from professionals.
I didn't really talk until I was 7 years old.
I did speak a little but it was VERY limited and only to my mother and father, I never spoke in school, to strangers, or to the wider family - I went to a speech therapist for a long time after I started talking because my speech was very poor. Even then I chose not to speak most of the time, my mother had to talk for me, if I needed anything I got it myself rather than ask...and everyone refused to babysit me as I never talked and it used to freak them out
I just didn't speak, it never occurred to me to speak because a. I don't think I fully grasped that other people were other people like me with thoughts and feelings, b. I didn't see a need to speak because I got by fine without talking, and c. I didn't understand why people interacted with each other like that as I was fine talking to myself - not sure if others ever did this, but I would have conversations with people in my head and that was enough to sustain me, I didn't actually need to talk to other people. I was a little shy, I didn't know how to speak to other people or how to make the words come out. I've also always been ridiculously stubborn so for some time I didn't talk because I didn't talk and didn't like people trying to force me or trick me into talking.
It's hard to say why I didn't talk as it was a long time ago, my memory is bad, and I really find it VERY hard to understand why I was that way, I was so much in my own head that other people and even things happening with my own body just didn't register.
Yeah its something like what you posted I'm sure. What you say about being 'so much in my own head', I think he's like that. I'm NT but I was always in my own head day-dreaming..it was so bad the school told my parents I must be ret*d because I wasn't learning and insisted on tests. I had to teach myself to read and write because I didn't pay attention. My letters to this day are upside down. Similar things with maths.
I do suspect he's like me, he's in his own head, but then he also has Autism too.