Diagnosis of diabetes needs to be accomplished chiefly by blood studies.
If this boy presents difficult to manage behavior, it is likely not related to the diabetes (if he actually does display diabetes).
Hopefully the boy's mother will seek medical evaluation - the impact of diabetes can be very serious, and it would be wise to rule it out medically even if the mom has no worry.
I COMPLETELY DISAGREE WITH THE DOCTOR. My son's behavior problem is the only reason I took him to the doctor to be tested for diabetes. He is a very loving, funny little person but, one day while arguing with his father he reached up and grabbed his dad by his collar to argue with him.....I knew something was wrong. He had had behavior problems for about two years at school, I overlooked it as just a growing boy. But, when his behavior changed with his loved ones, I knew something was physically wrong with him. He is a type 1 diabetic with a thyroid problem. I will never again look at kids as bad kids, until the doctors clear them of all physical problems.