Unwanted Group Message

1. Go back to your list of active iMessage conversations by tapping the blue arrow in the top left.
2. Find the group chat
3. Swipe left on the group chat
4. Tap Delete

Follow any further on-screen prompts to delete the conversation from there.

Do this on both you and your sons phones. Then create a new conversation with just him in it.

Let me know if the steps are unclear, happy to clarify if necessary.

You cannot remove group participants from a group with only three people in it.

The best way to resolve this would be to delete the group on you and your son's phone. Then start a new conversation just with your son. Siri should now send messages straight to your son rather than the group.

See if Siri still does the same if you make the request more specific. For example, say to Siri "Send a text to John Appleseed" and then when she asks what you would like the message to say, speak your message. She'll ask for confirmation, then say to Siri something like "Yes, send it."

See if Siri still creates a group after trying specific requests like this.

That's strange. Check your sons contact in your iPhone, maybe you have accidentally added a second number to his contact.