When my daughter was taken in to see our GP she did a finger prick test to measure her blood glucose. She was confused by the result because the meter said "Hi" and she thought perhaps the meter was faulty because usually she said it displayed a number. Now that I'm more experienced I know the meter read Hi because her blood glucose levels were so high it was off the scale of what the meter could measure.
Because of the confusion a urine test was also done to measure ketones, which came back positive and we were sent straight off to A&E. It is possible that if you are catching the diabetes early on the urine test may not show as positive. It's not the most accurate way of testing.
What sypmtoms is your child showing that makes you believe your son has diabetes? My advice is that if you feel the doctor has missed something then get a second opinion. Ask for a blood gluose test to be done rather than a urine test. Doctors can make mistakes or miss things, best to get it double checked if you're still worried.