@csherman66
Does your grandson carry his water cup with him all day? If he does, you can try to have it in one place in the kitchen so he has to ask for water each time. Also, changing to a regular cup usually cuts down on the volume.
Years ago it was common to give toddlers bottles or sippy cups with apple juice- often diluted.
I was guilty of that! They became dependent on sweet liquids and would cry for more and more. This bad habit stopped when it was found that many children were nutritionally deficient.
Excessive thirst is also connected to another form of diabetes called diabetes insipidus. It is a very rare condition and stems from hormonal problem in the pituitary gland. Your daughter could ask the pediatrician to test for that as well as testing kidney function.
I hope you will post again !