Trust me when I tell you that there is life after diagnosis. I have been diagnosed for 13 years and am now 55. I still take my dog on long walks, enjoy family activites and lead as normal a life as possible.
Of course, the muscle stiffness takes its toll, but a positive attitude and the will to fight against the symptoms is essential. PD is not terminal, whilst it can be very dehabilitating, it will not cause death in itself.
I am sure your Dad knows how lucky he is to have you around. Be strong for him and show him that you dont see him as ill, and that life goes on regardless. It is more challenging that it used to be, but use this to find out what he really is capable of.
Its a case of tailoring your life around the PD rather than letting it take over. Make sure you allow your Dad to be as independent as he can be, whilst helping him when he asks for it. He is the best judge of how he is feeling and what he can do.
It is important to treat him and react to him exactly as you always have done - with or without PD he is still the Dad you love and will continue to love regardless of how the PD develops. If you can be strong for him, that will help him immensely.
Good luck and keep your chin up - its not all downhill from here, life will hold many challenges, but face them head on and remember to look after yourself as well as your Dad. Carers tend to be forgotten, but they need support as well, its not easy to watch someone you care for deal with PD symptoms.