What is the fate of a man who repented and became a caller to Islam, yet has a non-Muslim son from a previous relationship in his youth, and what can he do to atone for his sin?

If a man repents with sincere repentance and performs righteous deeds, Allah will accept his repentance and replace his evil deeds with good ones, and he will become beloved to Him and brought near to Him. He must beware of the whispers of Satan and rejoice in Allah's grace upon him through repentance. As for the young man's claim of fatherhood, it is baseless, and there is no legal relationship between them, nor is it permissible for him to adopt him. However, it is permissible to treat him kindly, raise him, educate him, and invite him to Islam.