Hi,
Very sorry about your brother and your family's loss. This is surely a very hard time for you all.
Sounds like he died of diabetic ketoacidosis. You can look this up. The high blood sugar causes severe dehydration. This condition can progress very rapidly once started. Blood sugar of 600 is highly dangerous and if not treated quite quickly will be fatal.
For the rest, we cannot answer. He would not have felt anything once he was already unconscious.
I hope you find peace.