Supplements / Protocols to slow Parkinson’s?

Hi, My dad (60M) was recently diagnosed with Parkinson’s disease. At the moment, his symptoms are very mild, his main notable symptom being his movement has slowed down over the last couple of years before his diagnosis, but other than that he has no tremor etc. He is currently looking into clinical trials etc, and has started exercising a lot at the gym - a mix of cardio, resistance training and stretching - as we have read this can help a lot. He also has a very healthy diet of mainly whole foods, plant-based with very little meat and takes a few supplements (creatine, multivitamins, omega-3s). However, I wanted to ask if any of you are aware of any supplements or protocols which have been shown to / may help with the slowing down of the progression of the disease?