how do you allow my son to add friends across platforms?

Hello paweljaworski,

Thanks for reaching out to us, at the current moment you cannot add friends from other platforms on your Xbox friends list without them first having a Microsoft account. You can connect other social media platforms to see if any friends on those platforms
have a Microsoft account. I'll post a link below showing you how to connect other social media platforms to your Microsoft account.

Linking Together : https://support.microsoft.com/en-us/help/4028404/microsoft-account-linking-accounts

Glad I could provide you some details for your question, let us know if you have any other questions or issues.

Happy Gaming!

Aside from the other helpful responses to this question it may be worth checking the game/game developers website for more information regarding cross-play.

it is usually the game that you would do that. make an account to the game developers like epic game, EA games, or riot games etc... and should be an option in the profile to link your console.

Hello paweljaworski,

Thanks for posting on the Xbox forums. We understand that you are interested in learning how to allow your son to add friends across platforms. This process is usually completed in the game itself. The game will have unique IDs or accounts that allow
users to add each other as friends while they are playing the game. If your son's Microsoft account has any sort restrictions on our end, you can adjust

the online privacy and safety settings for his account.

If the privacy settings for your son's account allow him to add friends, you will want to reach out to the developer for the game. They will be able to provide more information about adding friends on a different platform through the game. Be sure to
let us know the results paweljaworski!

In order to play Fortnite across multiple systems, your son and his friend have to

link their Gamertag and PSN account to an Epic Account on the Epic website.

After that is done, they will have to add themselves as friends on the Epic website.

Then, when in the Fortnite game, your son needs to press the Menu button and navigate to Epic Friends.

If your son is under the age of 18, you might have to sign in with your parent account at
https://account.xbox.com/settings, choose your son's account, and set up his online and privacy settings to allow him to play cross platform.

This page here also has information on
how to crossplay Fortnite across different platforms.