All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
Convincing a Muslim of the impermissibility of something should be done by highlighting the Sharee‘ah evidence from the Quran and the Sunnah that indicate its impermissibility. A Muslim should principally comply with the Sharee‘ah and submit to it, but the greatest obstacle that prevents him from such compliance is the pursuit of his personal inclination (desires), which can be treated by intimidation and warning. You should contemplate how Allah, The Almighty, warned the believers after underlining the enjoined submission to Him in the following verse: He Says (what means): {It is not for a believing man or a believing woman, when Allah and His Messenger have decided a matter, that they should [thereafter] have any choice about their affair. And whoever disobeys Allah and His Messenger has certainly strayed into clear error.} [Quran 33:36]
Therefore, you should strive in advising your son and warning him of the Wrath of Allah, The Almighty, and His Punishment and remind him that He may punish him for listening to what He has prohibited and that the punishment may be inflicted on him in the worldly life or in the Hereafter, which is severer. Also, you should not neglect supplication, which is a great means to rectify and reform one’s children, which many parents are heedless of. Allah, The Exalted, Says (what means): {…and make righteous for me my offspring.} [Quran 46:15] Moreover, the Prophet said: “Three supplications are answered, and there is no doubt that they are answered …[he mentioned] and the supplication of a parent for their child.” [Abu Daawood, Ibn Maajah, and others]
One of the most important ways as well to rectify your children’s conduct with regard to listening to music is to eliminate the means leading to it by moving them from the non-Islamic school that teaches music.
Allah Knows best.