Suggest Treatment For Frequent Urination In A Teenager

Hi My 18 year old son is complaining of excessive urination. The symptoms started approx a year ago. It took him to a urologist in area. Fluid intake output record showed that he was drinking 3 liters of water everyday. He reduced it to 1 litre. Lab results ruled out diabetes. His dad has type 2 diabetes. The symptoms went away for some time. A couple of months ago I took him to Urology clinic at South Western Medical Clinic in Dallas. They didn t see anything wrong. He was fine couple of months but since yesterday he is again complaining. He said he woke up 5 times at night 2 nights ago. Last night he woke up only once. His urologist did the sonogram of his bladder and didn t find anything. He is about to embark into college life in Aug. I am at a loss now. Need expert advise please.