What are our duties toward a son who uttered indecent words—not intending to mock the religion—while teaching the Qur'an, and is his speech considered a minor sin or a major sin?

Using foul language that includes obscenities in a مجلس العلم (gathering for seeking knowledge) is considered ill-mannered and a violation of the texts that command guarding the tongue from evil and obscenity. The Prophet, peace and blessings be upon him, ordered the guarding of the tongue in many hadiths, among them his saying: "Whoever believes in Allah and the Last Day, let him speak good or remain silent," and his saying to Mu'adh: "Are people thrown into the Hellfire on their faces except for the harvests of their tongues?" And he forbade obscenity in speech, saying: "The believer is not a slanderer, nor a curser, nor an obscenity-user, nor an evil speaker."