I hate to hear your having a problem sending and receiving messages from one person. I know how important text messaging can be. Let's look into this immediately.
Do you or the person your texting have a signature on your text messages? If so please remove it. A signature could be a smiley face or " sent from ...'s phone".
Next please delete the old text messages from the person your having the issue with. Then reset the iphone. Here's a link that will show how to reset the device. Don't worry this reset will not cause you to lose any personal information.
Please also try adding a 1 to his contact information and retesting. Let me know if these steps help.