He vowed to break the fast in Ramadaan

Firstly: fasting Ramadaan is obligatory for adult, sane
Muslims who are able to fast. If a person meets those criteria, it is haraam
for him to break the fast with no excuse. It is also haraam for him to swear
to do that, because by swearing such an oath he is resolving to do something
haraam. 

Secondly: 

If a Muslim swears to commit a sin, it is not permissible for
him to do what he swore to do, rather he must break his vow and offer
expiation for breaking an oath (kafaarat yameen), because the Prophet
(peace and blessings of Allaah be upon him) said: “Whoever vows to do
something then sees that something else is better than it, let him do what
is better and offer expiation for his vow.” Narrated by Muslim, 1650. 

Kafaarat yameen (expiation for breaking an oath) means
feeding ten poor persons, or clothing them, or freeing a slave. If a person
cannot do any of these things, then he must fast for three days, because
Allaah says (interpretation of the meaning): 

“Allaah will not punish you for what is unintentional in
your oaths, but He will punish you for your deliberate oaths; for its
expiation (a deliberate oath) feed ten Masaakeen (poor persons), on a scale
of the average of that with which you feed your own families, or clothe them
or manumit a slave. But whosoever cannot afford (that), then he should fast
for three days. That is the expiation for the oaths when you have sworn. And
protect your oaths (i.e. do not swear much). Thus Allaah makes clear to you
His Ayaat (proofs, evidences, verses, lessons, signs, revelations, etc.)
that you may be grateful”

[al-Maa’idah 5:89] 

See also question no.
45676 

Thirdly: 

The one who has done that must repent to Allaah, for it is
very repugnant for a Muslim to swear to break the fast in Ramadaan. This
indicates that he is careless about the sacred limits set by Allaah, and
that he takes them lightly. In the answer to question no.
38747 we stated that it is very
dangerous to break the fast in Ramadaan with no excuse, and that the one who
does that is regarded as a hypocrite, Allaah forbid. 

Al-Dhahabi said in al-Kabaa’ir (p. 64): 

Among the believers it is well established that the one who
fails to fast in Ramadaan although he is not sick and he has no excuse that
would permit that, is worse than an adulterer or one who is addicted to
alcohol; indeed they would have doubts about his Islam and would think that
he is a heretic and a deviant. End quote. 

We ask Allaah to keep us safe and sound and to make us
steadfast in adhering to His religion. 

And Allaah knows best.