I am sorry to hear about your dad's symptoms. Postural hypotension can be quite debilitating, as it sounds it has been for your father. In order to provide recommendations on the best course of action, it is necessary to know the full details of your father's medical history and to perform a thorough physical exam. I strongly encourage you to schedule an appointment with a cardiologist
Typically, upon standing, gravity causes blood to pool in the legs. Normally, to counterbalance this, your heart beats faster and your blood vessels contract in order to maintain a normal blood pressure. Postural hypotension describes a finding of low blood pressure upon standing. This often occur due to dehydration and can cause loss of consciousness. Excessive pooling of venous blood in the legs can contribute. An inability of your heart to compensate also may play a role. Finally, certain medications can inhibit the compensatory response of either your heart or your blood vessels.
The first steps in treatment are to eliminate medications that may be contributing and to ensure adequate water and salt intake to maintain a normal circulating blood volume. Fludrocortisone and midodrine are often used, although it sounds like these have not helped your dad. Prescription compression stockings can also help prevent too much blood from pooling in the legs. I encourage your to see a cardiologist to determine the next best course of action.