The loss of vision is due to the the development of diabetic retinopathy
Blood vessels get damaged in diabetic retinopathy resulting in proliferative retinopathy or macular oedema.The treatment for diabetic retinopathy includes meticulous control of blood sugar,blood pressure and blood lipid levels along with laser treatment
People suffering from diabetic nephropathy
are put on ACE INHIBITORS or ARB's for control of blood pressure.
If these agents are not prooved to be successful in bringing down the blood pressure,then alpha blockers or even centrally acting agents may be used.
You must consult your doctor for the prescription of proper anti hypertensive medications and with proper judicious administration of these medicines,the blood pressure can be properly controlled.
Along with anti diabetic medications and anti hypertensive medications,your father must observe proper dietary restriction along with required exercises.
I hope it helps.