I recently encountered someone who was able to start the quest without the message. Some other guides also mentioned the same thing.
xRyudsaid: Yesterday i finish doing underground pass, i talk to the king and just go to do something else waiting for the king's message to start regicide. I've waited +3hours and didnt received yet. I read in reddit, that now i dont need to wait for the message, just need to keep doing the quest so i continue questing,
You might want to go straight to King Lathas and see if it works for you.