As sallamu alikum, my dad has change his religion from being Musli...
Wa laykum salaam:
Here is an answer straight from our Qur'an:
2:256 (1) There is to be no difficulty in the Standard. (2) Certainly, the conscious realization (or understanding) of the temptation (seducement; seduction; allurement) was apparent. (3) Therefore, whoever rejects the tyranny [(طغوت tağut) the false gods] and believes in Allāh, (4) then certainly he should grasp the strong handle. (5) There are no breaks [Lit.: cracks without separation(s)] for it. (6) Allāh is Unlimited Hearer, Omniscient.
[In other words, what's there to argue about? It's apparent that whoever forsakes false religion, polytheism like all current forms of Christianity are for example, and truly believes in God, the Omniscient Hearer, should know to take hold of the strong and everlasting handle of the Word of God.]
If you wish to check my translation here it is in Arabic:
لَآ إِكۡرَاهَ فِى ٱلدِّينِۖ قَد تَّبَيَّنَ ٱلرُّشۡدُ مِنَ ٱلۡغَىِّۚ فَمَن يَكۡفُرۡ بِٱلطَّـٰغُوتِ وَيُؤۡمِنۢ بِٱللَّهِ فَقَدِ ٱسۡتَمۡسَكَ بِٱلۡعُرۡوَةِ ٱلۡوُثۡقَىٰ لَا ٱنفِصَامَ لَهَاۗ وَٱللَّهُ سَمِيعٌ عَلِيمٌ (٢٥٦)
(1) .256 Lā ikrāha [√krh if’āla, acc., m., impf. act., Stem IV] fīd-dīni (2) qad tabayyana [√byn tafa’’ala, 3rd, m., sing., perf. act., Stem V] r-ruşdu [√rşd] mina l-ğayyi (3) fa-man yakfur biţ-ţāğūti wa yu’min billāhi (4) fa qadi istamsaka [√msk istaf’ala, 3rd, m., sing., perf. act., Stem X] bil-‘urwati [√’rw] l-wuŧqā (5) lā infiṣāma [√fṣm infi’āla, acc., m., Stem VII] la-hā (10) wa llāhu samī’un ‘alīmun
From an Islamic perspective then, don't worry about it. No one is supposed to shove religion down another person's throat, especially if they want to be faithful to God. When or if he ever wants to learn true Islam (Lit.: "Surrender!" or "Be safe!" or, as commonly translated, "Submit!"), teach him what he wants to know and leave it at that, but remember that mankind cannot "join" God's religion, but mankind is commanded to "islam" [if'al, an imperative of the root S-L-M, to submit, to be saved or safe, to be at or make peace] to the standards of God, meaning that we have a choice to obey God by embracing His Standards like the day we were born or reject the standards of God. Your role in the drama between you and your father is to be the example, the living beacon you are called to be, for him to follow.
Let's clarify something though so that, regardless of what School of Thought you subscribe to, you do not fall into the very trap of joining or creating sects of Islam such as Sunni, Shi'a, Ismaili, etc., which is clearly forbidden in the Qur'an. The religion of God is not a social organization that requires any type of ritual to join like baptism, tithing, or accepting Muhammad as your Lord and Savior as the Christians do with Jesus or the Baha'is do with Baha'u'llah. It's a Standard mankind is commanded to submit to. Only God can be a member of his "religion"! The Qur'an very seldom speaks of the "religion" of God, but what is translated erroneously as "The Religion of God" actually means "The Standard of God," the ensign that flags what we are called to be, do, and embrace. HOWEVER, the word for religion in Arabic is used in referring to the devotion of Abraham. Now, because all things are decreed by God, trust that God has His purposes for locking your father's heart at this time and for however long his heart is locked, whether we know that purpose or not.
Kindly let me know if this helps.