These medicines are not responsible for increase in blood sugar levels.
It has happened because of failure of beta cells in the pancreas
As the diabetes
progresses in type 2 diabetics,the mass of functionally active beta cells in the pancreas starts dwindling and soon a stage is reached where the mas of active beta cells is reduced to a minimum and this is the stage when oral diabetics loose their efficacy and they become ineffective even at the highes possible dose.This is the stage where injection insulin
has to be added and your father seems to have reached this stage.
You should talk to your doctor and get some injection insulin added to the treatment regime.
Once insulin is added,your father would have much better control of sugar levels.
It is quite remarkable that your father has kept his sugar level normal by use of oral anti diabetic drugs for 40 years.