I used to get a lot of pain, particularly in my right leg. Often paracetamol was quite effective. If that didn't work, I took Ixprim (= tramadol + paracetamol). Not everyone with Parkinsons has pain, but if you do, then it is very bad. The pain clinic had me taking four ixprim a day whether or not i felt pain. That worked for most of the time, if it didn't I could take up to another four; but there were other consequences. So I then had to take Lopraz and Motilium as well. Plus regular kidney checks.
Then my neuro got involved. The pain is Parkinson related. To describe it inaccurately but simply: the brain it not receiving messages from the leg, so it worries and tries to check on the leg; this results in pain. All pain is actually in your brain. If your little finger signals to the brain that it has been burnt, then your brain creates the pain, to draw your attention to it. Similarly, in a minority of people with Parkinsons, the brain creates pain when it is not in full control of you leg (or arm etc)
My neuro said that the pain was the result of the lack of dopamine. He dramatically increased Stalevo. He was right. The pain dramatically reduced. It still made its appearances, but it was less frequent and significantly less painful. The regular ixprim were dropped.
Last June I had a serious fall and did some serious damage. I was (and still am) in a lot of pain for more conventional reasons. Now, I don't know if this is permanent, but although there is a lot of "conventional" pain, I don't detect that parkinson pain. Possibly my brain is so occupied with damage reports from my spine, that checking the leg is less urgent?
Summary: your Dads pain is (imho) parkinson related, it is caused by the lack of dopamine. The appropriate solution is to take artificial dopamine. Being in constant pain can really destroy your life. Sorry for rambling on so much.