iMessage is not working for this particular contact? Does he have a good data connection. If he sends you a message, do you receive it as an iMessage? Try deleting messages from this contact and then have him send yhou a message and see if you receive it as an iMessage, and if so, when you respond does it go as an iMessage?
On Both Devices...
Close All Open Apps... Sign Out of the Account... Perform a Reset... Try again...
Reset ( No Data will be Lost )
Press and hold the Sleep/Wake button and the Home button at the same time for at least ten seconds, until the Apple logo appears. Release the Buttons.