How can one deal with an atheist son who denies the Sunnah and the commands of religion, describes the family as extremist, and wishes to marry a Muslim woman, knowing that he refuses discussion and justifies his atheism with materialistic and scientific reasons?

The parents are not sinful if they strived to raise their children upon Islam, and then one of them deviated, for the guidance of hearts is in the hand of Allah. The parents must continue to supplicate for their son for guidance, for the supplication of a parent for or against their child is answered. They should also not help him with his marriage or meet anyone concerning him as long as he is in this state. They must continue to advise him and seek the help of a specialist to converse with him.