I never agree with beating a horse, but remember you are dealing with a 1200 pound animal. For the most part, nothing you do to him will inflict much real pain.
This is my favorite saying: "As soft as possible, and as hard as necessary." Cherie is right...spank his butt. Perferably with something like Parelli's carrot stick, which seems to be a electric fence poll with leather wrapping on the end. A whip is too small and will sting a little too much, as least that's what I've found. Start with lounging small circles around you, just out of kicking range, in case he gets fresh. I'm talking maybe 10 to twelve feet in diameter small. Start with light taps, and if he goes, great! If he doesn't, tap harder. That's how my horse was...after a minute or two he went "fine! I'll go forward!" With the small circle you'll start with, a trot is really as fast as you want to go, and remember you can always let out more line if it's going good.
When I say tap, that's what I mean. I practiced on my leg to make sure my taps stung, but didn't really hurt. You want to get his attention, and stinging his backside will do that, but you don't want to just start wailing on him.
With my horse, I spanked him on the muscled part of his back leg above his hock. Kinda midway between his butt and his hock. This gave him reason to move forward, and didn't hit him anywhere where the isn't much muscle. And with Raja, it was about irritating him with the number of taps. And after about twenty seconds of hard smacks (my hard taps were about like a girly swing at a tennis ball--mostly from the elbow, with my upper arm at my side) your arm will burn, but he might be going, "ya right, that don't hurt," but keep at it. And remember that your stick can be used as reward to "rub away the smack."
Basically, ground work is a great way to get to refocus him on you and not on the scary outside world. Doesn't matter if it is a hay trailer or dump truck or the garbage man, he needs to focus on you. And the best way to do that is to lunge him, and make him move. Frequent changes of direction (reversals) are good too, but can be tricky when you have line and stick in your hand.
Keep in mind this worked for my horse and might not work for yours...you know him better than me!