I hope I do not seem hopelessly arrogant and ignorant with my response. I have no formal psych. training. Am a lawyer looking into becoming a play therapist.
When I read about the boy and the seemingly very good environment he is in plus the parent's anxiety, I immediately thought that the boy must be struggling with some very subtly traumatizing behavior by the mother or perhaps the father, or both. Maybe the boy is incorporating some distorted reaction to the world - or to HIM - that he then experiences in unpredictable situations.
Hope that layperson's thought might be helpful.