Will Downloading Minecraft Beta remove my normal version of Minecraft?

Yes this is from the beta trying to integrate with the normal minecraft. Once you opt out of beta, you have to uninstall beta version and install the normal version again.

I wanted to update my Minecraft Windows 10 but when updating minecraft I always download the beta version of the game

I have found a solution. After I deleted my original insider account I created a new one and joined the rtx minecraft beta other than the nether beta which was what I was in. I then updated minecraft to rtx and unenrolled and reverted back to the normal
minecraft version.

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.

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.