Post the trot and use your rythm to speed him UP! Sit less and post up/forward more pronouned. As soon as he does speed up, scratch his withers and say "good boy" and relax your posting. Rinse and repeat until he gets it. Keep your legs well under you and squeeze with your leg (bumping might be startling him). And keep your leg on him at all times (lightly), so when you do ask him for more, it's not such a surprise. Keep your toes in and squeeze with your lower leg.
Also, try some voice cues, like clucking.
Sit UP, shoulder's back, BREATHE, soft back and hips, LOOK UP, soft hands, and think FOWARD!
You also might look at your saddle fit & placement. I know my mare doesn't like to trot out if I don't have her in her favorite saddle and pad combination, lol. When I ask for more trot, she'll try to canter, or throw her head up, or both! lol Make sure the saddle tree is behind the horse's shoulder blade (under the front d-ring on an English saddle). Make sure it's sitting even on his back (not too high or low on one end). Put it on without a pad and check underneath for air pockets or tight spots. You should feel even pressure front to back.
My mare seemed to improve when I bought a ThinLine pad
. It's a thin peice of dense open cell foam that eliminates pressure points, absorbs shock, and distributes weight very well. I use the standard black pad, not the Ultra thin or cotton or sheepskin pads. One nice thing, they have a 30-day guarantee, so if you don't like it you can send it back. My mare really seems to like it, and the saddle stays put better (she prefers a loose girth), so for us it was worth the money.
You might think about some Dressage lessons too. Dressage (classical anyway) is all about FORWARD! A fresh perspective might be just the ticket to help you two get over this "hump" in your training.
ETA: I rode a horse once who also bucked everytime you cropped him. No, it wasn't pain, he was just a brat! lol I don't know if you want to try what we did (my trainer supervised and instructed me), but everytime he bucked, he got whacked hard and kicked. Toward the end of the lesson, we were in a bucking-kicking-smacking match, but it finally ended with him giving up. He never bucked at the crop again 8).
That said, a crop is not necessary to make a horse go forward. Your leg and seat can do the job just fine ;).