A healthy child's blood glucose
still fluctuates hour by hour during the day. Eating, exercising or an illness can cause sugars to increase or decrease. Normal values for a child without diabetes
are 70 to 130 mg/dL.
Your son's value of 5.1 before bed translates to 92 mg and is therefore normal.
I don't think there is any need to go for any urine test.
However I would advise you to discourage your child from loading up with unnecessary sugars in any form.