Texts

Hello,

An Apple ID is meant for just 1 person’s devices meaning that you should not be sharing an Apple ID with their iPad. Checkout this Apple Support document on how to setup their own Apple ID. You would then restore the iPad to factory settings and set it back up with their Apple ID.

Family Sharing and Apple ID for your child - Apple Support

Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support

You have a couple of options. As stated in the response above, you really shouldn’t use more than one device per Apple ID and the only way to guarantee that he won’t get those texts are to assign the iPad a different Apple ID. However, you can turn off text forwarding in the settings on the iPhone to stop it from sending the texts to the iPad, but this is no Guarantee that he won’t still get them in some sort of way.

Your best bet is to assign a new Apple ID to the iPad and then turn on Family Sharing which will allow him to still be able to have access to most apps that you purchase on his Apple ID as well.