Can’t install apps on Family Setup Apple Watch

bought an Apple Watch for my son and set it up under his Apple ID using family setup from my iPhone. He does not have an iPhone of his own. There are various parental controls in place through Screen Time. The watch works but he is unable to install apps using the App Store on the watch. He can browse the App Store but when he clicks “Get” to download an app, he enters his passcode, then his Apple ID password, but the app fails to download. It just goes back to the app screen where he clicks “Get” again, and goes through the same loop. We did try disabling the “ask to buy” option under family sharing, but this did not change anything. I cannot install apps using the Watch app from my iPhone, because this is not an option when using “family setup.”