My son also drinks alot in a day. I talked to his doctor about it and they say as long as it's not all sugary drinks they are fine. Mine will drink water, milk, juice, cool aid, or capri suns all day. Sometimes having 2 sippies in 15 mins. So relax, it's ok..
I`m not so much worried about types of fluids but we have diabetes in our family. I know excessive drinking can be a sign, but how much is classed as excessive?