Have you tried a backpack with a book in it? Or just a bit of weight? If you are going to the store or the neighbor's or something he would then put the backpack on... that bit of weight keeping him focused. My son wears a monkey leash (controversial I know!) but I hold the little harness part of it because otherwise his impulses have him literally running everywhere and grabbing and dumping things, crazy really. Well if I need him to be extra calm for a short while, I put a little weight in the backpack part of the monkey... I'm also going to order a "chewlery" for him (that might not be spelled right) but it is is a little rubber chew thing that you can clip onto clothes. His need to bit and release that muscle pressure seems to keep him calm / less impulsive.
I think the trick is... that you need to make a short list of a few things that keep your son from being so impulsive. When is at his best? And how can those things then be translated to the real world.