Thanks for the info, Luckypenny.
A week?? I guess I won't get too freaked out yet. I'll see what the vet says when I call tomorrow. I have to make an appointment for booster shots at the end of the week anyway.
I know people on here recommend wet food a LOT. But when you get a new kitten from a shelter environment, and you have to do a two week quarantine for health reasons from your other cats - it just makes sense to not change too many things all at once. One the kitten is deemed fully healthy and can integrate with the rest of the house, that's the time to tinker with food. We had a kitten a few years ago who we had to try several different foods before we found one that didn't give her an upset tummy. If you change a bunch of stuff in the first week, you have a hard time knowing if diarrhea is from new food or worms or something else.
I'm mostly just happy this is his only problem. He's a real cuddler, but he also has tons of play in him. He's already brought a lot of joy into our lives.