Thor seems to be all better. He completely his Flagyl course yesterday, he has not been vomitting, and his stools remain normal. He's also gone back to being a lazy bug in the morning.
On the bright side, I've found that he really loves Breathies Minis, and he only needs a tiny piece to be happy.
I am going to make an effort to stop feeding him some of the richer foods he gets. We do a lot of trick training, and I sometimes need something he's really excited about to get him motivated for a difficult trick. But Breathies just might be that treat.