iMessage Won't Work with Only One Contact - Works With All Others

I'm able to iMessage all of my iPhone contacts except one, which shows up as a text message. Both he and I have tried restarting iPhones, restarting iMessage, reentering our Apple IDs in iMessage, and also reentering our contat info, but the problem still exists. No problem with data plan on either end. Both of us can send/receive iMessages from other iPhone users in our contacts. Anyone know how to fix this? We are both using iPhone 5 and the latest iOS version.