This is neat because I just came across the same situation =) I've been working with my new yearling colt on walking on a leadline because he's only been on one twice before I brought him home. He learned in one day how to walk, and whoa, at my voice command and by just watching my motions. He is super smart! However, I can't for the life of me get him to trot on command...lol same thing as you, i'll be the one trotting ahead while he just walks behind me looking at me like i'm a freak! I even tried really lifting my knee's high so he would see what I was doing and he still looked at me like a freak!
I think it just takes time and a lot of reassurance on their behalf. They're taught to be gentle around humans and to move slowly, so to suddenly be asked to perk up and trot around you is different and awkward! Just reassure him that trotting is what you want him to do, voice command is a great idea, say "Trot" in a crisp, clear, medium-loud voice (not sweet, soft and coo-like!) and pull on the leadrope a bit so he understands you want him to move faster...just a little pressure, not yanking. Remember, a good lesson taught is a good lesson remembered! and always always end on a good note with him so that is the last thing he remembers about you.
Good luck!