Dot,
Frogs tend to stay in the water for comfort when they are sick. My first thought if he has loose, white, watery stools is that he has a parasite infection.
The Frog First Aid article by Paul Rust has treating info:
"Listless, often found in water or hiding, watery droppings.
Parasites in intestines, “Protozoa”
Parasites are always present in the intestines, but they will cause problems when the animal is weak. Treat with METRONIDAZOLE (FLAGYL)
1 drop in 8 drops of tepid water. Place one drop on the back of frog. Duration: Each day for 6 days. Treatment for the vivarium: 1ml per 5 liters of water; use this to spray the vivarium."
I would seperate him from the others (quarantine) and also treat him with a tepid water and honey solution, which will soothe and help rehydrate him. You can also read about this in the article listed on the left site navigation bar.
I hope he starts feeling better. Keep us updated on how he is doing. One of the experts can pipe in and correct me if I'm wrong. But it wouldn't hurt him to try this treatment. Or if you feel he is getting worse, then take him to a vet.
Wishing you well.
~Lesley