Trying to join the insider beta wont work

if any one of them has the insider minecraft beta installed it wont allow them to join

Your son is playing on the official release channel for Minecraft. His 1.12 version is currently the latest official version, whereas his friends have joined the Insider Program and are playing a beta test version of Minecraft.

His friends can either un-enroll from the Beta Insider Program, or you son can try to join the Insider Program and update his Minecraft to a beta version.

Be aware however, that beta version still contain untested parts, and the game can crash, malfunction and in turn destroy worlds if an error occurs.

If your son is still willing to test the Beta, he can download the free Insider app from the store, and join the Minecraft Beta.

Then he'll have to download/update his Minecraft to the newer beta version.

You can either join the Insider Beta as well and get the 1.13 beta, or you can ask your friends to leave the beta, uninstall Minecraft, and re-install the official 1.11.4 version.

If you wish to join the Beta, you can open the Windows 10 Store, and search for the Xbox Insider App.

Install the App, and sign in.

Under Insider Content you should see the Minecraft Beta. There you are able to join.

As Ackis already said: since this is an old thread, please create your own new thread.

In the mean time, ask the friend you're trying to connect to, if they joined the Minecraft Beta Insider program. if they are using a beta version that is newer than your regular version, you'll get that error.

Either ask your friend to leave the Insider Program for Minecraft, or join the Minecraft beta program yourself by downloading the Xbox Insider App from the Marketplace.

To become an insider, download the Xbox Insider Hub app on your console. You join through the app. After that, you can join various betas.