Hello,
Welcome to the ehealthforum and I am really glad to help you out.
If your father is having diabetes and is taking medicines then he has to take these medicines regularly after getting a through examination and his blood sugar and HbA1c levels done. He might need dose adjustment of his medicines as well.
For regular estimation of his blood sugar levels, you can purchase a glucometer and check his blood sugar levels on a regular basis when he feels weak. He may be having hypoglycemia which can turn dangerous.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care.