Hello, I have a Unlimited smartphone plan that is connected with a separate line for sharable data with an Android Tablet. I noticed the tablet has a text messaging app pre-installed on there. Does the sharable tablet access plan allow for unlimited text messaging, or do they charge a "pay-per-use" fee to send out texts?
A couple of things to consider:
1) Assuming that you are tethering your tablet to your phone (i.e. using your phone as a wifi hotspot) you can check your cellular data plan to see if it supports unlimited texting (both national and international). If it does then you should be good to go. When in doubt try a test of one or two messages and then check your monthly bill to see if you were charged for the service.
2) A popular alternative is to use an instant message app such as WhatsApp or Signal. These apps are free to install and free to use. Both parties (i.e. you and your son) must have the app installed to communicate. During installation the apps will request access to your contact list.
WhatsApp is owned by Meta (Facebook) and is immensely popular.
SIgnal is an independent entity and, while not as popular as WhatsApp, is considered by many to be more secure.
Wikipedia link for WhatsApp is here -> WhatsApp
Wikipedia link for SIgnal is here -> Signal