Type 2 HbA1c – what does that mean???

My husband’s diagnoses is Type 2 HbA1c 13.7% Diabetes mellitus. We are learning and could use the help of this forum to understand … What do these numbers mean? I see others posting something similar but do not understand what this means in terms of "insulin resistant" or diet needs.
He is currently in hospital (one week now). He had very high bg levels (500+) and was placed on a diet of oatmeal and mineral water with natrium (salt) plus meds of metformin (500mg 2x) and torasemid (diuretic) + insulin injection mid day. BG is now at 146 mid day reading and he has been placed on a normal diet. No more insulin injections, continuing with met and the diuretic.
Is this type of diabetes controllable through diet and the met? Is that what "insulin resistant" means?
Initial diagnoses was 3 weeks ago and I am reading as much as I can to become educated.