OS X Mavericks :: Changing Contact Email To Phone?

I have a friend where I have a single convo with and a group convo with via iMessages. There were times where his messages were sent from his number, and other times from his email address. As you may know, on the Mac it keeps it in the same window. However, on the iPhone it'll separate it. So he unchecked his email on his iPhone. So now everything will be in one window with him on my iPhone, as well as the 3rd friend.  

But now on my Mac. I can not use the group convo to send him a message. I still receive messages from him in the window that I've had opened for the last year. But I can not use my Mac to send a message to the 2 of them, because he won't receive the message.  

With that being said, on the single convo. I was able to click his name and a little window pops open. I was able to select his phone number. It automatically switched over so my messages are now being sent to his phone number. It even displayed "iMessages sent to *phone number*". 

However, on the group message. I select his name and the little window only shows the fact that his email address is no longer registered with iMessages and asks me if I want to remove it. No matter what I do, I can not see his phone number as a selection. Nor can I permanently remove his email address from the group convo. It will disappear, but once I click out of the window then back, his email re-appears.  

I've tried quitting Messages, rebooting the Mac but I can not change his contact info in the group's convo. I don't want to delete that message just to start a new one. Unless I really have to