I agree, I did half water half juice until my son was 4. Straight juice would always upset his stomach.
Quoting TinkerBell2013:
The drs told me to take and add water to the juice if the child gets a diahrea from drinking it. It always helped with my boys. They cant drink juice without getting a diahrea.
I would try to get him to drink pedialyte. They will keep him from getting dehydrated.