I work with a standardbred/Perch cross from the Amish at a therapy barn, I've also worked with a number of draft horses bought at auctions in Penn. I'm not going to get into that.
As for being jittery, yes many of those Standardbreds are quite hot, the Amish actually need to abide by a speed minimum on many streets to avoid causing car accidents and such, so the horses need to be hot. Which is how our therapy program got that cross, she wasn't fast enough. Many of the horses are encouraged and expected to move at least at a fast trot all the time. So this will take time to unteach your horse. Most, when they realize they don't need to run, are relieved. It just takes time to let them realize.
I find many Amish broke horses are very rein-reliant and really have little to no understanding of seat or leg cues. Obviously there are exceptions, but as most are primarily cart or plow horses, riding skills aren't commonly reinforced. So remember to sit heavy, soften your body, and lots of half halts to help ease him back to a more comfortable trot. You'll spend a LONG time going through that, release pressure when he's trotting calmly and lots of half halts, heavy seat and calm body to keep him soft. He'll probably go very well on voice commands. Often they use "G'ip" and "woah", try to avoid using the word "Easy" as most horses are put on edge by the sound of that word, "eeeeassssssy" it's just hissy. A low, deep "woooaahh" is more effective.
They also tend to have little to no bendabilitiy, they'd rather take big wide turns, tight turns confuse and make them uncomfortable. So this will probably take time for him to learn too, don't expect him to be spinning or anything any time soon :P
Sounds like you have a good fun horse, but quite a project ahead of you. It just took a month or so to get the therapy horse up to par, but she was a lazy girl, not a hot one like yours.
The draft horses I've worked with I drove most, most didn't know how to ride but would tolerate someone sitting up there and steering them. They're very used to the shafts hitting their sides so they don't really understand leg commands at all, since they've been trained to ignore it. The love of my life Belgian I got riding great in just a short time, but again a lazy type.
Good luck, keep us posted and We Love Pics!