Welcome, bradd!
I believe this question has been asked before.
The novel had no reference to Michael and diabetes. The only reference to any illness was a notation that Michael had been discharged from the Marines in WWII after suffering "a debilitating wound." In GFII, he swallows a pill during his train ride from Tahoe to Miami to see Roth. We don't know what the pill is for. A guess: diabetes was treated with insulin injections in those days. I'm guessing it was a Miltown, the popular tranquilizer of the era. (Senator Geary popped one, too, in Michael's office.) But, if memory serves, others here seemed to find hints of some illness earlier on.