How to Block or Mute Group Chats in WhatsApp, iMessage, Instagram
How Do I Unblock Someone from a Facebook Group?
How to block FaceTime spam group calls
How to Block a Group Text on an Android Device
How to See if You've Been Blocked on Facebook Chat
How to Block Someone in Facebook Chat: 10 Steps (with Pictures)
Until then you can use the solution given above.
When he turns on Chat, you will likely regain the ability to send and receive messages with him in real time.
Group chats are designed to reach and communicate with more of your friends easily.