Do you unlock new features,mobs,blocks,etc from the Minecraft beta insider update?

According to https://minecraft.gamepedia.com/Bedrock_Edition_beta_1.13.0.1 it seems you opted in to recieve BETA updates to Minecraft.

As Beta versions are test versions and might contain unstable code, public servers do not update to Beta versions.

So if you wish to use public servers, make sure you only use non-Beta versions of Minecraft. If necessary, un-enroll from the Minecraft Insider Beta program, and wait until the next official update, or uninstall and re-download the latest public non-beta
version of Minecraft.

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 could try to put half blocks down. Or any other block that mob can't spawn on. Glass, Glowstone,etc.

Are you in the Xbox insider program?

If you are, then either leave the Minecraft beta downloading the Xbox Insider Hub from the Windows Store. Then, open the app, go to the Insider content section, click on Minecraft for Windows 10, manage and unenroll.

Alternatively, your friends can follow the same steps and enroll in the beta.

If you're not in the Xbox Insider Program, make sure both you and your friends have the latest version.
Open the Start menu and open the Store app. Next, click your user picture and choose Downloads and Updates from
the drop-down menu. Hit the Check for updates button to run a check for new versions of your installed apps.

It's a glitch that came along with the new lighting system. Find the block that's producing the most light, place a block on it and then mine the block. The light should then go away.This glitch also makes making mob traps frustrating since the light will
keep mobs from spawning.