maybe he drinks lots of soda and stuff like that, have him drink natural water, instead of colored water or sodas, change the diet, no more candies or unhealthy food and see what happens...
my daughter used to have dark urine, she would go to school and hold her self as long as she could, sometimes only going once during her 7 hr day at school, till i started asking questions and figured out she wouldnt drink but a couple sips of water at school and a couple sips of water from her water bottle at lunch , not what they should drink a day,