Most ingested molds do not cause any severe problems. If he were exposed day in and day out to mold, then I would be concerned. However, since it just happened yesterday and he hasn't vomited or developed any other symptoms, I would not be too worried. Call your pediatrician tomorrow just for safety's sake. In the future, you will want to use a little wire brush on those cups and the valves. They are also okay to boil usually.
I hope this helps. Let me know if you need more info. T
It is possible that he is experiencing symptoms resulting from mycotoxins which can be produced by mold. It can cause food poisoning symptoms such as nausea and vomiting.
It is okay to keep him at home tonight as long as the diarrhea is not severe. You can give him pedialyte and water. Stay away from soda and fruit juice b/c they tend to make diarrhea worse. He can also have very bland foods. BRAT- bananas, rice, applesauce, toast.
Still be sure to call your pediatrician tomorrow. The diarrhea may not be related to the mold at all. It could just be a virus, but that is your physicians call.
Monitor his hydration, by making sure he has tears when he cries, his mouth is moist, he is alert, and is urinating at least once every 8 hours. If these things start to lag, he needs seen.