In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
2- But if he prayed sitting down because he was unaware of the Shar`i ruling, and he thought that because it was permissible for him to sit down that he could say the opening takbeer whilst sitting, then he does not have to repeat the prayer.
Answering your question, the Fatwa Center at Islam Q and A, states:
The scholars have stated that it is obligatory to stand for the opening takbeer in the obligatory prayer.
An-Nawawi (may Allah have mercy on him) said in Al-Majmu‘ (3/296):
“It is obligatory to say the opening takbeer correctly, pronouncing all the letters, whilst standing. If the worshipper pronounces any letter of it when he is not standing, then he has not started to pray.”
Al-Akhdari al-Maliki said:
“The obligatory parts of the prayer are: the intention (niyyah) to offer a specific prayer; saying the opening takbeer (takbeerat al-ihram) whilst standing; reciting Al-Fatihah whilst standing; bowing…”
Al-Kharashi (may Allah have mercy on him) said in Sharh Mukhtasar Khaleel (1/264), discussing the obligatory parts of the prayer:
“Standing for the opening takbeer in the obligatory prayer, for one who is able to stand and is not joining the congregational prayer late, is obligatory. Therefore it is not acceptable to say it whilst sitting or leaning forward.”
In al-Mawsu‘ah al-Fiqhiyyah al-Kuwaitiyyah (13/220) it says:
“It is obligatory for the worshipper to say takbeer whilst standing, in prayers in which it is obligatory for him to stand, because the Prophet (blessings and peace of Allah be upon him) said to Imran ibn Husayn, who was suffering from haemorrhoids: “Pray standing and if you cannot, then sitting, and if you cannot, then lying on your side.” An-Nasaa’i added: “… and if you cannot, then lying on your back.” Standing is done by making the back straight.
So it is not permissible to say the opening takbeer whilst sitting or bending forward, and when standing is mentioned, it includes that which may take its place, such as sitting in obligatory prayers when one has an excuse.”
The basic principle with regard to the prayer of one who is sick is that he must do what he is able to of the obligatory and essential parts of the prayer, and whatever he is unable to do is waived in his case.
So if he is able to start the prayer standing, he must do that, then if it becomes difficult for him to remain standing, he may sit down.
In Al-Fatawa al-Hindiyyah (1/136) – which is a Hanafi book – it says:
“If the sick person is unable to stand, he may pray seated, bowing and prostrating [from a seated posture], as it says in al-Hidayah.
The soundest of these scholarly views concerning the meaning of incapability is that what is meant is if he will suffer harm as a result of standing.
But if it is somewhat difficult for him to stand, it is not permissible for him to refrain from standing, as it says in al-Kafi.
If he is able to stand for part of the time, but not to complete it, then he should be told to stand as long as he is able to, even if he is only able to say the takbeer whilst standing, and is not able to stand for the recitation, or if he is able to stand for part of the recitation but not for the entire duration thereof. In that case he should be told to say the takbeer whilst standing, and to recite as much as he is able to whilst standing, then to sit down when he becomes unable to remain standing.”
Based on the above, your father has to repeat this prayer in which he forgot to stand for the opening takbeer, if he was aware that it was obligatory for him to stand for the opening takbeer.
But if he prayed sitting down because he was unaware of the Shar‘i ruling, and he thought that because it was permissible for him to sit down that he could say the opening takbeer whilst sitting, then he does not have to repeat the prayer.
Almighty Allah knows best.