All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
Sanity is a condition for the validity of fasting. If a person has mental disorders because of insanity or old age, then he does not have to fast or pray, nor does he have to provide food to make up for the fasting that he misses. A person has to feed instead of fasting if he, while having a sound mind, is incurably sick or too old to be able to fast. This is because sanity is a requirement for legal accountability, there is no accountability in the absence of mind.
If this is the situation of your father, then he neither hast to fast nor feed. Yet, you are allowed to provide food on his behalf (if you wish to do so).
In contrast, if your father has a sound mind but is not able to fast, then he is required to provide a mudd of food per day of fasting that he misses in charity. A mudd of food is about 750 gm of rice or the like. The amount of wheat that is mentioned in the question is more than what is required. The questioner may ask her father’s permission to buy the food with her own money. She also has the choice to provide the food day by day or all at once at the end of the month. However, she is not allowed to provide the food in advance.
We ask Allaah, the Exalted, to reward you and your father and to help you remain dutiful and benevolent to him. Your father needs you, and you are advised to read Allaah’s saying (which means): {Do not make [as equal] with Allaah another deity and [thereby] become censured and forsaken. And your Lord has decreed that you not worship except Him, and to parents, good treatment. Whether one or both of them reach old age [while] with you, say not to them [so much as], "uff," and do not repel them but speak to them a noble word.} [Quran 17: 23 - 24]
Allaah Knows best.