What Causes Reduced Urine Output In A Child?

My son is 5 years old, he s constantly asking for drink but hardly has to urinate. It goes as far as he will drink right up to the time he goes to bed, he never wets the bed and you would think he would make a bline to the toilet to receive himself when he Wales but he goes to the fridge for a drink instead he won t pee for at least 3 hours of waking.