Your description of the symptoms of your father suggests that he was being treated for Parkinsonism and diabetes.
You have mentioned that he had slowness in his activities, which was asymmetrical to start with (that is, one side was more affected than the other). However, most of the Parkinson’s disease patients have asymmetrical tremors at onset (involuntary shaking of fingers and hand even at rest), which you haven’t mentioned. Nevertheless, since he was started on anti Parkinsonian medicines by a doctor suggests that he was suffering from Parkinsonism. It is important to note here that Parkinsonism and Parkinson's disease are not the same entities. Many patients with features like Parkinson’s disease (which include slowness of gait and activities, stiffness of body, tremors and inability to adjust to postural changes) but having other different symptoms and signs may be erroneously diagnosed as cases of Parkinson’s disease.
The hallmark of Parkinson’s disease is deficiency of a brain chemical called Dopamine. Doctors therefore try to supplement it by giving either Dopamine or other drugs, which increases the action of Dopamine in the body (called Dopamine Agonists). Sometimes drugs, which delay the breakdown of Dopamine in the body, are also used (which thus make available more Dopamine that is produced in the body). The two drugs namely Amantrel and Selgin, which your father was receiving, fall in the last two categories.
These drugs although very useful in the initial years of treatment may later on produce side effects. These include unpredictable response, loss of effect and troublesome additional movements called dyskinesias. These effects usually develop after 5-7 years of treatment especially with dopamine supplementation (Levodopa tablets). Although your father didn’t take levodopa, dopamine agonists also at times can cause such effects.
More likely explanation of the current state of your father however would be the progression of the disease itself. These diseases are degenerative diseases and progress slowly despite treatment. Unfortunately, we still do not have drugs, which can reverse the changes.
I strongly recommend that you consult a good neurologist.