I posted this another time, somewhere. It gives a good explanation of different executive functions:
http://files.campus.edublogs.org/bl.../List-of-Executive-Functions-169lmt8.pdfMy lay analysis would be it sounds like it's an issue with "Shifting." I don't think that's uncommon, or particularly alarming. Like Pemberly explained--I think a lot of creative people go into a zone sometimes. Csikszentmihalyi would call it FLOW.
It might help to identify this for your DS and see if he can think of some strategies to help--there may be a bit of Metacognition at play, too. If he can begin to identify what triggers the "creative shocks" he might have more success stopping them from progressing when they are inconvenient. It could just be something simple like "close your eyes and shake your head" or "get a drink of water," etc. Or say to himself "time and place" or something coping related like that.
I relate to him (minus the creative piece), being the world's worst multi-tasker. It's uncomfortable to get lost in thought when you're not supposed to be.