imessage is only for idevice to idevice and will not send your son's group messages to those without idevices. It uses imessage to send messages unless the person who is receiving the message can not receive it due to network issues or the reciprient choosing to not have it turned on (as in the case of those who do not want to exceed their data limits due to excessive texts). As he already knows. It uses data rather than text message in terms of your network billing. There is nothing that will remind him to turn it back on unless the realization of green bubbles instead of blue ones does.