Hi, there could be many causes however to check his blood, it should be checked two hours after he has eaten or had anything to drink.
Good luck
No, there is no other reason to have high blood sugar like this except diabetes.
I am sure you did not want to hear this, but sounds like your 8 year old is diabetic. This can be very dangerous (even life threatening) if not treated. I'd recommend you get your son to his pediatrician immediately and tell the pediatrician what you've just told us.
No one wants their child to have diabetes, but the sooner you get this diagnosed and managed the better for everyone.
Please don't allow him to drink sugary drinks and eat sweet foods which will make things worse. But get him to the doctor urgently.
Please let us know waht happens wth your son.