What Causes Bad Odor And Severe Thirst In A 7 Year Old?

Hi, my son is 7 years old. He seems to be thirsty all the time. (which I have just thought it was normal for an active little boy). His breath has this oder, even after he brushes his teeth. Today I finally did a finger prick and tested his blood sugar. I did the reading when he got home from school around 3:15 (3 1/2 hours after lunch) and it read 423. I had to retest it because I knew that was way to high, and it read 350. We took him to the doctor and they checked his urine, which was clear, and did another finger prick. This time it read 98. So our doctor sent us home telling us we probably got a wrong reading and that it happens all the time. I am concerned because he did not do a Gtt or any other testing. My son s breath problem has been going on for some time now along with the thirst. He has no weight loss, but eats ALL the time. Is it possible for a child to NOT be a diabetic and their sugar level spike that high and then come down and have clean urine? Or does these blood sugar levels mean that my son is probably a diabetic? I dont understand why my doctor did not do a Gtt.