How do I permanently delete social media apps from a child's iPhone?

The same way you delete any app. Press and hold on the App until the menu pops up, and select remove App, then delete App.

click here ➡️ Delete apps on your iPhone or iPad - Apple Support

With that said, if you want to prevent him from downloading them again, disable the App Store in Settings ➡️ Screen Time ➡️ Content & Privacy Restrictions ➡️ iTunes and App Store Purchases ➡️ Installing Apps

Make sure there’s a password set for Screentime, and that your son does not know it.

click here ➡️ Use parental controls on your child's iPhone or iPad - Apple Support

In addition, after you have deleted the Apps, you are going to want to prevent them from reinstalling them. You do that by going to Settings > Screen Time > Content & Privacy Restrictions > iTunes & App Store Purchases > Installing Apps, and make sure this is set to Don't Allow.

You are going to need to use a Screen Time password they do not know and it will prevent them from installing any apps. This will give you complete control and if there is an app they want to download, you will need to do that.

Many Social Media apps also have websites where they can access their account as well. You can also use Screen Time to prevent access to websites, but honestly it is not 100% effective. There are other websites that act as portals to the restricted website for the sole purpose of bypassing these restrictions and those websites that change frequently will also need blocked.

Screen Time is just a tool to help in many situations and for a child that truly wants to defy your strict orders, there is always a flip phone for only calls and texts. It is amazing how cooperative they become when showing up to school with a flip phone.