What Causes Excessive Thirst In An Autistic Child?

My grandson is 2 and a half years old and has a bottle that contains water at night when he lays down. He is autistic and has sensory issues. His pediatrician is not concerned that he has a bottle to lay down, but we had him last night and we re alarmed by his excessive thirst. We have him frequently during daytime hours and he is constantly drinking water out of a cup, but we viewed this as a good thing. I have enquired recently, because he is drinking so much at night that he urinates through his diapers at night and must be changed approximately 4 times each night after he lays down. Last night was the first overnight experience for us, and he drank 5 8oz bottles of water and drank one right after another. He was so thirsty, as we were grabbing him another bottle, he was drinking out of his cup. Is this a sign of a medical condition?