Before blood glucose meters were widely available, people with diabetes would test to see if they were high or low by doing urine tests with chemical strips- kinda like the way we test for ketones these days. The problem with this is that the strips weren't extremely accurate and it was a better indication of what your blood glucose level WAS about an hour ago.
So every time your son has a high blood sugar, his urine will have some sugar in it. He probably had higher than normal BG (blood glucose) levels when he had his urinalysis and that's what the pediatrician saw. This would have been a decent indicator of a diagnosis of diabetes, but you already knew that. In terms of being an indicator of how good his overall control is the A1C score is the best measurement, as it is an average his general BG range over 6 months. 7.9 is a fantastic number and you guys deserve a pat on the back for maintaining that level of control. It's hard work.
One last thing: remember what I said about the old-timey urine strips to measure blood sugar levels? This is amazing: before the urine strips were invented, doctors would taste the urine for sweetness to diagnose diabetes.
my blog: http://parentsguidetodiabetes.blogspot.com/