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, sallallaahu 'alayhi wa sallam, is His slave and Messenger.
Any Muslim who neglects fasting in Ramadan without a sound reason commits a major sin, and he should first repent to Allah because he has violated Ramadan which is a pillar of Islam.
Abu Hurayrah narrated that the Prophet said: ''Whoever breaks fasting a day of Ramadan without an excuse that Allah has set out for him, then he will not be able to make up for it even if he fasts for the whole life.” [Abu Daawood, At-Tirmithi and Ibn Majah]
Moreover, Imaam Ath-Thahabi said: "It is established among all Muslims that whoever breaks fast in Ramadan without a legal excuse, such as an illness or a travel, is worse than an alcoholic, an adulterer and a fornicator.” They even doubt his true belief and suspect him of infidelity and decadence. Moreover, he should make up for any days he missed to fast. The evidence for this is the order given by the Prophet to the man who had sexual intercourse with his wife in the day of Ramadan. He ordered him saying: "Fast a day instead of it " [Abu Dawood and Ibn Majah]
Since it is obligatory for sick people and travelers, who are legally excused, to make up for the days they missed to fast, it is at least obligatory on those who did not fast although they were fit and not traveling, to make up for the days they neglected to fast. On the other hand, one is not obliged to make up for the days he missed on a consecutive basis unless the number of days he is supposed to fast and the number of days remaining before the coming Ramadan are the same. In this situation one must fast successive days for the Hadeeth narrated by ‘Aa’ishah who said: "Sometimes I missed some days of Ramadan, but could not fast in lieu of them except in the month of Sha'ban." Sa’eed Yahya, a sub-narrator, said: "She used to be busy serving the Prophet." [Al-Bukhari]
Therefore, delaying to make up for the missed days without a legal reason until the coming Ramadan is not allowed. But if one happens to do so, he should make up for the missed days and feed a poor person for each missed day as expiation. This is narrated from Ibn ‘Umar, Ibn ‘Abbass and Abu Hurayrah . In addition, if one has sexual intercourse during daytime in Ramadan, he should expiate for that as described below:
Freeing a slave exempt from defects and if one can not do so, he should fast two consecutive months. If he can not do so, he should feed sixty poor/needy people as narrated by Abu Hurayrah in the following Hadeeth: "A man came to the Prophet, and said, "I am ruined." The Prophet asked, "What do you mean?" He said, "I had a sexual intercourse with my wife during Ramadan (while fasting)." The Prophet asked him, "Can you manumit a slave?" The man replied in the negative. He [Prophet] then asked him, "Can you fast for two successive months continuously?" He replied in the negative. The Prophet then asked him, "Can you feed sixty poor persons?" He replied in the negative. In the meantime an Ansari came with a basket full of dates. The Prophet said to the man, "Take it and give it in charity (as an expiation of your sin)." The man said "Should I give it to some people who are poorer than me and my family, o Prophet of Allaah? By Him Who has sent you with the truth, there is no family between Madeenah's two mountains poorer than me and my family." So, the Prophet told him to take it and provide his family with it.
Allah knows best.