Can he make himself stop if he *has* to stop? In school, for instance, would he be able to respond to a teacher's asking him to sit down and be quiet?
Yes.
ETA: Usually, though, he cannot, on his own initiative, stop... Someone has to say, "Uh, DS! sit down please." We used to say "Uh, pause it, DS, you need to pause it and go back to the story later because now you need to get your shoes on..." A hand on the shoulder, a squeeze, etc. pulls him out, too... Anything like that. However, he is frustrated he can't, himself, see he is having a 'creative shock' and be like 'uh not a good time for this right now, must stop' to himself.
