Can people still view my messages as i message on iphone?

I have an Iphone 4s, i can view all my friends messages as imessages (blue messages) expect my ex boyfriends, does this mean he has switched off his imessages? As anything i send to him now sends as a text, which i don't want, as i know imessages are free, plus i would like to see when my messages are delivered, is this because he has switched his i messages off? Can i don't still send i messages to him? Or is it something i have done?

It is been sent as an SMS or MMS message because you are messaging a non apple phone
Or the iPhone does not have Internet connection at the moment due to poor reception
The iPhone does not have iOS 5 Or later version of software.
Or this person may have iMessages turned off.
Or your Ex may have changed his number and email associated with iMessage which in turn would cause the green text since no iMessage exits with the number or email you are trying to iMessage causing it to be sent as SMS.