Well, I've been checking the skin on his shoulders for signs of dehydration and so far, he's perfectly fine.
I gave him the chicken & rice for dinner last night and he had one more loose stool before bed. He had some chicken & rice for breakfast this morning and now, 3 hours later, he still hasn't gone #2. Perhaps that means that the rice is doing it's job.
Anyway, I will be taking him to the vet tomorrow. I'm thinking that either the bug is resistant to the sulfadimethoxine or he's going to need to be on it for a little while longer. The good news is that it's not getting any worse. (fingers crossed!!)