How do I change the wallpaper on my sons Apple Watch?

Hi Apple_support_group_for_apple_,

Thanks for coming to Apple Support Communities for help today.

This is a feature your son should be able to customize watch faces on his own. Here is an article that can help: Change the watch face on your Apple Watch.

Any custom wallpaper would come from synced photos that you have synced to his Apple Watch through his account in your Apple Watch app.

Have a fantastic day!