I'm starting to get the feel of it. As I see it, with data roaming off, airplane mode on, wi-fi on, while wi-fi is available, my son will be able to use iMessage. If Apple servers are not available, "send as SMS" will work over wi-fi (right)? If he wants to text where there is NO wi-fi, he will turn OFF airplane mode, and will be able to send and receive SMS over cellular (but not picture texts since cellular data is off). iMessage will also not work over cellular (with data off). He has to be sure not to use the phone or answer it if it rings while airplane mode is off! Other than that, no risk of unexpected charges as long as data roaming is off. Is that all correct?
He will have wi-fi for much of his trip. The idea is that he have a backup if he is on the street and needs to communicate. He can forgo sending picture texts over cellular, since that will require cellular data, and I don't see a safe way to turn that on...
If he uses iPhone messaging while on wi-fi, and Apple servers see him, but can't find the recipient, does "send as SMS" result in an SMS being sent from my son's phone, or from Apple's server? :)