I am hoping that the doctor did a urinalysis in addition to a urine culture. The culture would have been the best way to lok for infection, and a urine analysis (often by dipstick) would have screened for sugar in the urine which could be a signal of diabetes. If the urine culture and urinalysis are both normal (negative), then that is pretty reassuring.
It is possible that he had a urinary accident that you are unaware of, which has now sensitized him to going frequently. If this continues or gets worse, a CAT scan or MRI may be needed to be sure that there is nothing anatomically affecting the urinary tract.
In either case, you should follow-up with your doctor by telephone or in person within the week. He may ask you to collect all of his urine and monitor his fluid intake. He may also do some blood work to be sure there is no other evidence of kidney problems.
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