Praise be to Allah.
Intercourse during the day in Ramadaan is one of the most serious of the things that break the fast. Whoever breaks his fast by having intercourse has committed a sin. He has to refrain from eating etc for the rest of that day, and he has to make it up and offer the burdensome expiation. The evidence for that is the hadeeth of Abu Hurayrah (may Allaah be pleased with him) who said: a man came to the Prophet (peace and blessings of Allaah be upon him) and said, “O Messenger of Allaah, I am doomed!” He said: “What is the matter with you?” He said, “I had intercourse with my wife when I was fasting.” The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “Do you have a slave whom you can set free?” He said, “No.” He said: “Can you fast for two consecutive months?” He said, “No.” He said, “Can you afford to feed sixty poor persons?” He said, “No.” … Narrated by al-Bukhaari, 1936; Muslim, 1111.
This applies if he had intercourse with his wife – so how about one who transgressed the sanctity of this month by committing zina – Allaah forbid.
The one who does that violates two sacred things: the sanctity of the month and the sanctity of a forbidden relationship. And Allaah is the One Whose help we seek.
This young man has to repent mightily to Allaah, and do good deeds, and do a great deal of righteous deeds, and keep company with righteous people. He should strive to keep himself chaste by getting married if he can afford it, or by fasting, for that will help him to be chaste and to avoid haraam actions.
What he should do now is to repent from that evil action, make up that day, and offer the burdensome expiation mentioned in the hadeeth quoted above. We ask Allaah to forgive him and us, and to accept his repentance and ours. Ameen.