Some people who have had diabetes for a long time will have worked out how much sugar is realistic for their diet. We recommend that diabetics have a high fiber and low fat diet with complex carbohydrates. Sugar, chocolate etc are allowed in moderation and ideally as part of main meals. The use of sugar boosts to stave off hypoglycaemia is common however if he is having frequent episodes then I would be worried that his insulin regimen is not quite suitable for him and it may need review.
With respect to his vision, I hope he has regular eye checks as visual problems can occur.
Pharmacist
MRPharmS. Worked as Community pharmacist for 16 years. Undertaking a second degree in medicine.