Is a father's question about the reason for his son not praying, by saying: "Do you intend for Allah to forgive you?" considered an assumption of Allah's role?

"Taa'li على الله" (presuming upon God) is to swear that God will not forgive a specific person, or for someone to say to another, "Do you intend for God to forgive you?" in a manner that dismisses the possibility of mercy. This is an attempt to control God, and it is impermissible. As for your father abandoning prayer, if it is not out of denial of its obligation, then he is not considered a disbeliever according to the majority of scholars. Strive to advise him gently and wisely, and avoid harming him.