I am currently having issues with iMessage. I cannot text my friend for some reason (iPhone user as well). When I hold the iMessage to send it as a text, it says "Send as Email" instead. However, I can send texts to other people with iPhones as well. I have checked my "Send & Receive" and my number and email are checked. I have also tried deleting my friend's email on my contacts but no difference, only the email registers to my phone when that person tries to contact me via facetime, iMessage, etc.
On the other hand, my friend can be texted by other iPhone users except me. She can text me, but I cannot reply as a text message. On her "Send & Receive" there is no "check" on her phone number but rather a loading sign beside it that doesn't seem to go away.
Best Answer
Create a new message from your iPhone and pick his phone number as the receiver (do not use the existing conversation). Then when you send him a message the option should become available.
The reason you cannot send your messages as text messages is because he probably messaged you with his email-address as the sender, not his phone number.