I feel I need to secretly read my teenager's messages and diary to know how he is. Is that okay?

Spying in secret usually destroys trust faster than the danger you are worried about. Teenagers are entitled to an increasing amount of private space, and feeling spied on makes them hide more, not less. The Quran warns: “and spy not on each other” (Surah al-Hujurat 49:12). The only exception is when there are clear danger signs such as drug abuse, serious bullying or an intention to self-harm; then safety outweighs privacy. Otherwise, choose the open path: set digital rules by discussion, get to know his friends, and ask after him sincerely. Tell him you take an interest because you love him, not because you suspect him. See the digital family for next steps.