Apparently too much sugar late in the day does nothing to improve my son's disposition. It dawned on me after a couple of days of horrid behavior that he'd been eating candy late in the day (his grandparents put ring pops in his Christmas stocking, and he loves them - AARGH). So I figured it was worth a try to cut that out and see if it made a difference.
We stayed away from sugary treats tonight, and while bedtime was still later than I'd have preferred (due in large part to the fact that I was washing sheets and had to wait for them to dry), in general there was a marked difference in J's attitude. He was still busy and squirmy, and had a couple of moments where he didn't particularly want to mind and I had to put my foot down. But he was improved enough from the last few days that I'm going to keep trying the "no sugar on school nights" theory. Except for those few moments of obstinacy, the bedtime routine was surprisingly low-stress.
So, for now we're going with a combo of mama being unshakably firm on the rules and swift to follow through with promised consequences, and limiting sugar on school nights (and probably weekend nights, too, just for the sake of consistency). We'll see if that helps considerably.