First question, is he getting your messages even though they go out as SMS? I ask this because when someone blocks you either purposefully or on accident, your texts to them will go out as SMS and not give you any notification that your message did not reach them. I have accidentally blocked people before. When I did that, I could still send to them with no indication that the message was failing to get them. They may have actually gotten my messages. I actually just now blocked a friend and sent him a message just to see if he gets it while blocked. I will find out soon!
Second question, has he checked his own settings? You mentioned that this doesn't happen with anyone else.
Try toggling off the setting to "Send as SMS" in Settings - Messages, then send your boyfriend a message. Does it go out as iMessage or does it fail? Toggle the setting back on after this and then try sending again. I have messaged people who were in an area with poor coverage. Those messages go out as SMS. That's normal. If your boyfriend is usually in an area with poor coverage, that could also be the cause of this problem.
The last thing to check is to make sure that your phone number is associated with your Apple ID. You should do this even though the issue does not happen with other people you send messages to.
Have you spoken to AppleCare about this?