I'm really sorry to hear that your father has been suffering with depression since his diagnosis.
Has he spoken to his GP /specialist or Parkinson's nurse about how he's feeling? This is always a good first step..
We also have a Depression and Parkinson's information sheet which looks at self-help and other treatments: https://www.parkinsons.org.uk/content/depression-and-parkinsons-information-sheet
And our Helpline team are on 0808 800 0303 and would be happy to talk things through with you and your father. We also have nurses on our Helpline team who can give you a call back. All calls are free and confidential and the line is open until 7pm this evening.
I'm sure it will help him to know you are there for him through this bad time. Let us know how you are both getting on.
Joanne - Digital team.