How can I help my friend who was excelling but then deviated, given that there is a glimmer of hope in him and his family's desire to help him, and what is the solution supported by the Qur’an and Sunnah?

Approach the young man and advise him, reminding him of God, clarifying to him that he has moved from light to darkness, and from elevation to lowness, degradation, and the scorn of people. Choose appropriate times and means for advising him, seek help from God and righteous people, and provide him with materials that address his deviance. True guidance is in the hand of God: "Indeed, you do not guide whom you like, but Allah guides whom He wills. And He is most knowing of the [rightly] guided."