family imessage chat needing to adjust to android

We've had a family imessage chat for years. My son just got a google phone, and even though he's attempted to start a new family chat from his phone (he has the same phone number as before), my iphone continues to automatically register this as an imessage and he's not receiving any of the group chats. How do I get my phone to stop doing that? Even if I type in each person's name individually, my iphone converts it to our former group chat as soon as I send the text.