I can't send anything but iMessages to one contact only

Hi, when i have no internet access, I can send text messages to all contacts (incl iPhone users), except one contact. When I try to send him a message I get the notification: Cannot Send Message - iMessage needs to be enabled to send this message. The other weird thing is that he's the only one of my contacts who receives messages from my email account rather than phone number.

Both my phone and mac are set to start new conversations from my phone number. I have also tried to switch iMassage off and on, signed out and back into messages on the mac ... none of those have helped.