Hi Asterisk,
Welcome to Neurotalk, you should get good advice here. I can't say I am "someone knowledgeable" but I would recommend cucumin. I have had PD for 19 years, and altho' I have tried many things, the one mainstay has been curcumin. Buy it with piperine (Bioperine) added, this makes it more bioavailable. I take 1000mg per day. it is very safe having passed 6 human trials up to 12 grams. It is a strong antioxidant, helps maintain the blood-brain barrier, and is a chelator of heavy metals.
Also tell your father not to panic about PD. He will have many years yet to enjoy life, PD is a slow developing disease generally. After 19 years, I still drive, manage to maintain a half acre garden, and in 2 weeks am travelling to Austria to start a 9 day cruise on the Danube, and visit Vienna and Budapest.
Ron