Thanks for the advice, everyone. I talked to the SPCA today, and the woman said they're always getting people calling saying their new cat kept them up all night, then calmed down. I also talked to a friend whose mother works in cat rescue, and a friend of my mother's who's a veterinarian, and they all said the same thing. There could of course be other issues, but everyone seems to concur that the likelihood is that he might do this for a few nights, but he'll calm down eventually.
Other than the night-howling, he's acclimated surprisingly quickly. He used the litter box right away, he approaches us to be petted, he plays with his toys (little catnip mouse on a string, laser pointer toy). He still goes and hides in the closet or bathroom sometimes, but then comes back out. I've left his carrier open, with a soft towel inside for him. He occasionally goes in there.
This cat does not like other cats at all. He was rescued from a hoarding situation, and the rescuers think he was picked on by the other cats. Apparently he's happier alone, though I imagine he's probably bemused by the sudden lack of other cats.
BF and I are going to play with him before we go to bed, then hope that he's settled in enough today to not yowl all night. And if he does yowl all night, we went and got earplugs