I'm not going to say you're wrong for handling things the way you are as I have been through this myself with my own son who is now 17.  Your objective is to encourage him to remain focussed long enough to complete whatever task is at hand, but at the same time, to allow him to reach this point without making him feel as though he is failing in some way.  Remember, this is a true medical condition he suffers from but one that can be improved with time and excercise.  Allow him extra time to complete such tasks as dressing by getting him started earlier and possibly encouraging him to lay out his attire ahead of time.  If he is too young to do this, by all means, help him prepare for the task, but not to actually complete it.  I know this technique has helped my son.  He now does all of this on his own, including getting his things together the night before.  He still needs longer to actually get ready, but he has learned to gather what he needs in order to not feel that frustration of "not knowing" and in order to make him feel as though he is not missing out on something.  That alone can be a distraction.  I tell everyone, and you will even read this in my responses to other posts, I strongly encourage video game playing as a tool to strengthen coordination and concentration skills.  HIGHLY RECOMMEND!  Believe me, it was made as a suggestion to us when my son was diagnosed and it has made a huge difference.  Please check my other posts for more information or feel free to contact me.  Good luck, God bless, and prayers for you and your son.