Everyone has his way of coping; denial is one that works for a while. One thing you can be grateful for with PD is that there really is no cure — the disease progresses no matter what you do — but there are things you can encourage him to do that will slow the progression of the disease. Encourage him to keep moving, get exercise, eat well (a Mediterranean diet is good), and stay involved with friends and family.
If there were a cure or even a pill that would significantly slow the disease he would lose opportunity for a higher quality of life through his denial, but since there isn’t, the disease will progress anyway and at some point he will give up on denialism and seek help.