Unable to receive text messages unless I'm connected to wifi (and receive them as imessage)!

Not true. I'm using iOS 16.3.1 and it is the same process to Unsend a message. Here is the support article showing how to tap and hold the bubble to bring up a menu allowing you to Unsend. In that same menu you can Edit the message and also "Send as SMS".

Unsend and edit messages on iPhone - Apple Support

While this option will work, I do agree that there should be a setting to prevent it. Your son may get 2 messages if the first one gets sent on iMessage, then the other as SMS. I don't see if it has been said if you shut off iMessage on your sons phone.

anibal234 wrote:

1. I don't think we're talking about the same thing. These are not MMS messages, these are just plain SMS messages. The problem here is that the sender iPhone knows that - at one point - it could send iMessage to a receiver so now it sends all the time as an iMessage but the receiver is no longer connected with data - so the iMessage is not delivered. The way to fix this could be:

If the message was not delivered via iMessage in a given amount of time, then switch it to plain SMS and send it over as an SMS

It does that if you turn on “Send as SMS” in Settings/Messages

2. Let the sender decide who to send iMessages to and who to send SMS to. That way at least I have an option - right now the ONLY option I have is to turn off iMessages completely on MY phone (the sender) so that I can send messages to my son. There's no way anybody else would know this or would be willing to do this to send an SMS.

You shouldn’t have to do that, because if you turn on “Send as SMS” that will happen automatically.

But the further complication is that SMS can contain only text, and is limited to 160 characters, or 70 characters if there are emoji in the message (because emoji require that the message be sent as 16 bit characters). If the message exceeds these limits or contains images or other files it will be send as MMS, but if the recipient cannot receive MMS (because they don’t have a a cellular plan) the message cannot be delivered.

anibal234 wrote:

How can I shut off iMessages on my son's phone? I'd be willing to try that, andI think we have tried already but I can definitely try it again. The option to tap-and-hold and then choose "Send as SMS" (or something like that) no longer works. I even went to the Apple Store and one of the guys there confirmed with me that this option is no longer present with the new iOS.

On your son’s phone, Go to Settings/Messages. Tap on Send & Receive, then tap on his Apple ID at the bottom of the page and sign out.

On sons phone: Settings > Messages > iMessage and shut it off. Also make sure you are sending to his phone number and not his Apple ID. Sending to his Apple ID is only available by using iMessage.

See my post on how to turn off iMessage; just turning off the switch does not disable iMessage.

Also, your screenshot shows MMS is on, which means that the phone has a cellular data connection. If this is your phone what you have to do is turn it off on your son’s phone.

Send the message then quickly tap and hold for a second the blue message you sent. An option will come up to send it as a SMS message.

Your son really needs a data plan. SMS messages use the voice channel, but MMS (picture and attachment) messages require cellular data. They cannot use Wi-Fi.

Apple can’t do anything about it. Carriers cannot send MMS over Wi-Fi because the cellular network doesn’t know anything about your Wi-Fi network.

I’m glad it worked out!