After we got home, I told him to take some Imodium.
He was chipper (and fever-free). No further dashes to the toilet.
I questioned him about his health scare, and he said, "Well, maybe it wasn't diarrhea. Maybe I just had to poop a lot. I'm sorry."
When I picked him up on Monday, I got a paper with this info circled:
Diarrhea and/or Vomiting: Student should stay home until they are free from these symptoms for 24 hours. Students who vomit at school will be sent home.
I let him stay home Tuesday, based on the above notice.