My dad suposedly cured himself of diabetes by eliminating all forms of sugar, except fresh fruit, and he started running marathons. He was diabetes free for approx 20 years, then it came back and he’s been on insulin now for 10-15 years. I’m gonna tell him about the cinnamon thing. That would be really cool if he could go off insulin again.
Find out if his C-Peptide level was checked. The pancreas makes proinsulin, which is cleaved yielding insulin and C-Peptide. IIRC, it's the easiest test to find out if a patient is insulin dependent.