IMessage not working with only one contact?

I have an iPhone 4, as does my friend. We had been imessaging when all of a sudden it wouldn't send. I double tapped send and set it to send as a text, but now it will not go back to imessage. I can still imessage everyone else, just not him and its driving me crazy! Any help would be appreciated. I've turned airplane mode and imessage on and off, restarted phone, deleted contact and message thread. Nothing works.

Imessage is undergoing maintenance.

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