Focus on being polite, formal, and supporting his ideas (even if they seem weird). Agree with Jumin and occasionally be "mean" to Zen to gain Jumin's hearts.
You need at least 50% chatroom participation daily to progress safely, though 100% is ideal for specific endings.
If you are looking for information on how teens interact online today or how to stay safe, here is the current landscape: Walkthrough - Mystic Messenger - First Day Chats
Balance joking around with him while also showing you care about his safety and serious emotions. 2. Real-World Teen Chat & Safety Guide