Welcome to HCM,
Your brother is diabetic and I presume this is Type I and you are right it is monitored by HbA1c
level and up to 7% is good control and beyond this level it reflect that blood sugar level is poorly control.
He should take care of diet and his insulin
intake at proper time,it requires lot of care and self discipline.
I think it is not the question how long complication may not come as acute complication and coma
can come any time if precipitated by any negligence and acute event like infection.
Proper understanding of disease is important and he himself has to take care, if not please discuss about insulin infusion pump
which may be costly but may help to control diabetes
In such disease patient and family cooperation is of utmost importance.
Please take care