Stop seeing phone messages on my ipad

My son can see my phone messages on his iPad. I need to stop this from happening

Hi. On your iPhone, turn off Settings > Messages > Text Message Forwarding.

Also, your son should log out of Messages with your Apple ID, and login with his own. If he is using your Apple ID, this is a bad idea for a lot of reasons. You should get him his own ID, and set up Family Sharing, if appropriate.