Send iMessage as text message

One of my best friends just switched from an iPhone to an HTC One X. When he texts me, I can't text him back because it sends as an iMessage and he doesn't receive it. How can I send an iMessage as a text message? I have iOS 7 downloaded onto my iPhone if that helps.
I've already tried to hold down the message for options but all it says is "Copy" and "More". When I tap more, it just selects the message so I can resend it. Nothing's working!

You have to go into settings and then go to your Message settings and sign out of imessage.

Your friends number is saved as iPhone in their contact instead of mobile. Go into your contacts, and change their number from iphone to mobile