I don't have any expertise in this area, but I did find a forum of people discussing similar experiences here: http://www.medhelp.org/posts/Neurol...eme-or-instense-daydreaming-/show/531449
Explanations suggested in that thread range from "maladaptive daydreaming", to "intense imagination/imagery with movement", which appears to be a type of complex motor stereotypy (http://onlinelibrary.wiley.com/doi/10.1111/dmcn.12518/abstract). My only other thought would be that it might be the type of things that's only a problem if it's a problem, if you know what I mean. So if he's experiencing distress and feelings of it being a disorder, it might be worth looking into further to perhaps see if you can improve his control over it.