It sounds like your horse is ignoring you or taking advantage of you. Especially if he keeps trotting during your downward transition.
I would first work on you. Practice imagery and energy. Picture your horse doing a lovely downward transition, listening to your seat instead of picturing him doing his usual dragging it all out. Change your energy! Be confident that he is going to listen to you. Don't think, "he's just going to trot a whole bunch before he walks and he's going to pull on my hands". Think "canter, canter, canter WALK" not "canter, canter, canter, trot trot trot trot trot trot, FINALLY walk".
Animals are amazing at picking up on our energy. That is their main form of communication.
As far as your riding goes, sit up tall yet sit deep in the saddle, sit just a touch back, think WALK, close your fingers on the reins and bring your elbows back a bit. You can keep some pressure on with your legs so you don't send him onto his forehand by just pulling back with the reins. You want to create a light downward transition that is still calling the horse to use is hindquarters.
Hope this makes sense! :)
Opportunity is missed by most people because it is dressed in overalls and looks like work. ~Thomas Edison