What is the ruling concerning a person who hasn't fasted one whole month of Ramadan many years ago due to some legitimate reasons and wants to make up for the lost month?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

You have to make up all those days that you did not fast after reaching the age of puberty. In addition, you have to feed thirty needy people for each Ramadan you missed fasting. The feeding of the poor people is an expiation due to the delay in making up the missed fasts during their specific time; i.e. eleven months before the beginning of the next Ramadan.

Know that if you had begun your day with the intention of fasting and then you broke it by having sex, then, in this case, you have to manumit a slave as expiation. If you cannot do so, then you have to fast two months consecutively, and if again you cannot do so [any of the above], then you have to feed sixty poor people as expiation. This ruling is reported in Saheeh Al-Bukhaari from Abu Hurayrah .

If the breaking of the fast was by any nullifiers of fast other than intercourse, the Muslim scholars have different opinions whether or not such person has to pay the mentioned expiation. Some scholars obligate the expiation while others do not obligate it.

Moreover, you should sincerely repent and perform as much good deeds as possible since you committed a great sin by neglecting one of the main pillars of Islam. The Prophet said: “Islam is built upon [the following] five pillars: testifying that None deserves to be worshipped except Allah and that Muhammad is His Messenger, the establishment of the prayer, the giving of Zakaah, the fast of Ramadan, and the pilgrimage to Makkah.”

Allah knows best.