I finally found something Flash hates more than being picked up - riding in the car. This kind of surprised me, given how laid back he normally is. I put him in the carrier with a towel, some hay, and a couple of little pieces of carrot, which is usually a good treat that cheers him up. He's had access to the carrier for a couple of days, and he goes in it to play, so he was already used to that. He just didn't like it when I picked it up, put him in the car, and started driving. He didn't even eat the little bit of carrot in the carrier.
So they weighed him at the vet's office, and it turns out he's smaller than I thought. Based on how big my sister's 8 lb bunnies were, I was guesstimating Flash at around 4 lbs, but it turns out he's only 2 lbs, 4 oz. He looks bigger, though, because he's long and skinny. But this means it's definitely time to cut down on the pellet consumption, and I think I should cut back a little on the fresh veggies, too. I was definitely spoiling him.
The vet thinks he's a young guy, probably only 4-5 months old and still growing. She said she wants to wait a month or two to neuter him, to give him more time to develop, even though it could be done now. But he's completely healthy - they even checked his poops to make sure he didn't have any parasites, and everything came back clean. Or as clean as poop can be, anyway.
Then came the ride home, when he just sort of braced himself in the carrier and didn't move again. When we got home, I put the carrier down and opened the doors (side and top), and he just sat there for a few minutes. Eventually, he started munching on the hay in the carrier, then wandered out to his litter box, and he seemed happier again. Given that the negative reaction lasted a few minutes after we got home, I'm wondering if maybe he got motion sickness from the car ride. I've never seen him react negatively to anything before, other than being picked up, but that's usually just a momentary panic until I put him back down.