I'd say if he won't believe he's not diabetic, it's not worth arguing (I have a 90 year old Mum like that!) - the thing to do is to say you've consulted people who are knowledgeable about diabetes and they say that fruit can be divided into:
Fruit with virtually no carbs which are suitable for diabetics - eg strawberries, raspberries, blackberries
Fruit which is OK for diabetics to eat in moderation - eg apples, pears
Fruit which tends to raise blood sugar too quickly so is less suitable for diabetics - eg bananas, exotic fruit, dried fruit
In reality it's a bit more complicated than that, and it does depend a bit on what each individual can tolerate, but most diabetics do eat some fruit. It's total carb count which he needs to watch if he is pre-diabetic (which is what it sounds like, if the 6.2 was an HbA1c test), not just sugar - a bowl of strawberries or even an apple is generally much safer for a diabetic than a bowl of pasta or a pizza, as well as being healthier generally.
If you want to check carb counts for individual foods so you can show them to him, I'd suggest using this site:
http://www.calorieking.com/foods/