Is it necessary for one who intends to come to Makkah and stay there for about a month before performing Hajj al-Mufrad (Hajj performed individually) to enter into ihram for Hajj from the miqat? And what is the ruling on one who passes the miqat without ihram and then enters into ihram for Hajj from Makkah?

Whoever intends to go to Mecca for Hajj or Umrah must enter into ihram from the Miqat if their place is outside of it. And whoever is closer to Mecca than the Miqat should enter into ihram from their own location, according to the Prophet, peace and blessings be upon him: "They (the Mawaqit) are for them (the people of those places), and for those who come upon them from other than their people, intending Hajj and Umrah. And whoever is closer to Mecca than that, then from where he originates."

If one passes the Miqat without entering into ihram, while intending Hajj, he must return. If he does not return and instead enters into ihram from a point closer to Mecca, then a sacrifice (dam) becomes obligatory upon him, due to missing the obligation of entering into ihram from the Miqat.

If your father entered into ihram for Hajj Al-Ifrad and entered Mecca, he remains in his ihram until he stones the Jamarat Al-Aqabah and shaves or shortens his hair. At this point, he completes the first stage of release (tahallul asghar), and everything becomes permissible for him except sexual intercourse. The major release (tahallul akbar) occurs after completing the circumambulation of Ifadah (Tawaf Al-Ifadah) and the Sa'i (if he had not performed Sa'i previously), after which everything, including sexual intercourse, becomes permissible for him.