Eye sight lost due to prolonged diabetic issues

my brother, a 60-year-old diabetic with Type II diabetes, insulin-dependent for the past 15 years. Two months ago, my brother experienced a sudden, significant loss in vision, resulting in cloudy eyesight and difficulty seeing his surroundings. After consulting with an eye specialist locally, he was advised to undergo vitamin therapy. However, the specialist indicated that his condition might stem from nerve compression or weakening of the optic nerves, which may not be reversible. We are eager to explore any potential treatments or interventions that might help restore his eyesight.