Hi, I am Van. I'm an Independent Advisor and a Microsoft user like you. I'll be glad to help you today.
This can happen if your child's privacy setting to add friends is disabled. Please follow the steps below to enable this setting.
- log in to xbox.com using a web browser.
- Once signed-in click your profile picture at the upper-right corner. Then select Xbox profile.
- In the Xbox profile click the privacy settings
- In the Privacy & online safety click your child's profile then click the "Xbox One/Windows 10 Online Safety" tab
- Scroll down and look for the "You can add Friends" option then click allow to enable it. Then click Submit to save the settings
- Run the game and check if he can now add friends.
You can also enable the "You can join multiplayer games" option so your son can play in Multiplayer mode
Please let me know if you need any further assistance, I will be glad to help
Best regards,
Van