Praise be to Allah.
The door of repentance is always open to a person, so long as he is still alive and the sun has not yet risen from its place of setting.
It was narrated that Abu Hurayrah said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Whoever repents before the sun rises from its place of setting, Allah will accept his repentance.” Narrated by Muslim (2703).
It was narrated from Ibn ‘Umar that the Prophet (blessings and peace of Allah be upon him) said: “Allah will accept the repentance of a person so long as the death rattle has not yet reached his throat.” Narrated by at-Tirmidhi (3537); he said: this is a hasan ghareeb hadith.
Allah, may He be exalted, accepts repentance from all sins.
Allah, may He be exalted, says (interpretation of the meaning): “Say, ‘O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful” [az-Zumar 39:53].
It was narrated from Abu Moosa, that the Prophet (blessings and peace of Allah be upon him) said: “Allah holds out His hand at night to accept the repentance of those who have sinned during the day, and He holds out His hand by day to accept the repentance of those who have sinned at night – until the sun rises from its place of setting.”
Hence it is not permissible to lose hope and despair of a person repenting.
As Allah, may He be exalted, says (interpretation of the meaning): “Indeed, no one despairs of relief from Allah except the disbelieving people” [Yoosuf 12:87] and “He said, ‘And who despairs of the mercy of his Lord except for those astray?’” [al-Hijr 15:56].
Despairing of the mercy of Allah, may He be exalted, is a major sin.
It was narrated from Fadaalah ibn ‘Ubayd, from the Messenger of Allah (blessings and peace of Allah be upon him) that he said: “There are three about whom you should not ask: a man who disputed with Allah regarding His cloak, for His cloak is pride and His garment is might; a man who has doubts about Allah’s decree; and the one who despairs of the mercy of Allah.” Narrated by Imam Ahmad (39/368); classed as saheeh by the commentators on al-Musnad; also classed as saheeh by al-Albaani in Silsilat al-Ahaadeeth as-Saheehah (2/81).
It was narrated from Ibn Mas‘ood that he said: The gravest of major sins are: Associating others with Allah, feeling safe from the plan of Allah, despairing of the mercy of Allah, and despairing of relief from Allah. Narrated by at-Tabaraani in al-Mu‘jam al-Kabeer (9/171); classed as saheeh by al-Albaani in Silsilat al-Ahaadeeth as-Saheehah (5/79).
Hence it is good for you to call this person to repent, strive hard to advise him sincerely, and show kindness to him by offering supplication for him.
Allah, may He be exalted, says (interpretation of the meaning): “And your Lord says, ‘Call upon Me; I will respond to you’” [Ghaafir 40:60] and “And ask Allah of His bounty. Indeed Allah is ever, of all things, Knowing” [an-Nisa’ 4:32].
Allah, may He be glorified and exalted, may decree wretchedness for a person because of a supplication, and He may relieve him of that because of a supplication.
What the people around this young man must do is be gentle and kind in calling him to come back to the path of guidance, and look for ways to exhort him, such as gentle words, good companions who will help him to do good and remind him of it, reciting verses of the Qur’an and quoting some hadiths of the Prophet (blessings and peace of Allah be upon him) that encourage people to turn back to Allah and repent to Him.
You should also exhort his parents and warn them of the danger of that; tell them that Islamic teachings forbid cursing a believer, for the believer is not given to cursing and slander; and tell them that cursing a believer is like killing him, as was soundly narrated from the Prophet (blessings and peace of Allah be upon him).
If cursing a believer is a major sin, even if that believer is a sinner, and it is not permissible to curse a specific sinner among the believers, then how about if that specific person is the child of the one who utters the curse?
And Allah knows best.