Answer
(Fatwa: 500/524/N=1433)
You should contact an authentic and reliable Alim of your area with whom your father has some attachment, and tell him the situation of your father, then that Alim should make your father understand the true religion in good manner and with wisdom and try to bring him on right path. Also you too should do duas for you father and also ask the pious and elder persons to make dua for him. In-shaAllah, your father shall receive guidance. May Allah guide all of us! Amen!
Allah (Subhana Wa Ta’ala) knows Best
Darul Ifta,
Darul Uloom Deoband