For your downward transitions, make sure that you're using your seat and legs as well as your hands. To transition, exhale, half-halt, slow/stop the motion of your seat (tip your pelvis forward, more on your crotch and less on your rump, this will put more of an S-bend in your back - all of this helps tell the horse, without reins, that the door forward is closing), close your legs slightly, and close the reins, in that order. Don't pull back on the reins, think more of ceasing to follow the motion of his head. (Western in a curb bit, I use a slight lift of my hand) You aren't pulling him to a stop or a slower gait/speed, you're just gently closing a door.
Something else to check into - if he's tossing his head up, something could be uncomfortable physically. Double check his saddle fit, his bridle adjustment, etc.
As far as walking and poo-ing at the same time, all you can really do is carry a whip or crop with you to back up your leg and just do your best to keep him going. Scout does the same thing, but I can count on one hand the number of times he's ever done his business under saddle in 2.5 years, so catching him in the act to teach the lesson is a little tricky.
Anyway, getting Buzz hot off your leg outside of his "bathroom breaks" will help your insisting mean more when the situation arises.