Rather than the reading, the need to escape/cool down/etc is a red flag (just my perspective)... many kids are tired at the end of the school day, but most kids don't need to "relax" when they come home from school at the end of the day. My 2e ds had a similar mode of operation (but different type of escape) when I picked him up from elementary school prior to his diagnosis and getting appropriate accommodations in place.
I can see where you're coming from, polarbear, and I guess it depends on whether this is a particular reaction to school, or a more general reaction. My DD is a textbook introvert and noise sensitive. A day in a noisy classroom sucks the life out of her, and she most definitely needs to find a quiet place to hide and re-energize after most days of school.
But she's been this way since a baby. She really enjoys people, but can only take so much noise and interaction before she needs to retreat. From the time she could make her needs known, when the house was full, there would always be a point at which she'd take my hand and pull me off into a quiet room somewhere to snuggle up and quietly read to her. When re-charged, she'd re-join. Those quiet spaces are essential to her. Ask her first thing on a Saturday morning if she wants to find friends, the answer will always be no. But ask at noon, and she's raring to go. She's recovered from the week at school by then.
I am not an introvert of this kind, and it has taken me a long time to understand that DH and DD need to be allowed to disappear in the middle of company sometimes, that it's not rude, it's just essential to their sanity. So if you think there's something about school or certain environments that is now shutting your son down in an unusual way, that should be pursued. But if that need for quiet breaks has always been there, that need to recuperate between exposures to loud crowded places, then I would see the retreat as pretty normal. It sounds like your son spends lots of time enjoying other people and activities, in addition to his reading. As long as there is balance, and he seems happy in both, he is probably finding the way of navigating the world that works for him.
