I've used the method the others have mentioned for many horses who can't\won't grasp the 'trot in hand'.
I use a longe whip that doesn't have a long tassle on it, and walk foward normally, ask the horse to "TROT", give him a chance to respond, then gently flick the hind quarter; Expect the horse to shoot foward the first couple times!!! THIS IS OKAY! Just run with him, praising him for his effort, then bring him back down to a walk, and praise him some more! Repeat, until you don't need the whip flick cue...some horses might only need a couple of times, where others you might have to do it a couple days in a row that way.
Have you tried desensitizing him to the whip, instead of just bringing it out, letting him see it, flip out, and taking it away?
"The ideal horseman has the courage of a lion, the patience of a saint, and the hands of a woman..."