I have a pre owned xbox console that's enrolled into the Preview Delta, How do I...

"Why no delta updates"

Sounds like your console is enrolled in the Delta Xbox Insiders preview. If so, you should check the Xbox Insider Forum on Reddit.

The reason you may be getting large 2GB updates is you missed a preview update and the console will download the complete updated OS specific to the Delta preview.

If your only internet access is a phone hotspot I suggest you unenroll from all Insider previews and return the console to the General release OS for xbox consoles.

For example, my console is enrolled in the Alpha skip ahead preview and I do 3-4 updates to the OS a week. It doesn't take long to use 2GB a week just in OS updates.

Has your son enrolled his console into the Xbox Insider Preview or is it a pre-owned console? I ask because there is a known issue regarding headsets in the Insider Preview build.

My Xbox is enrolled in the Xbox Update Preview in the Delta ring.

Its only Roblox.

Correct, I can open the game fine, but what happens after is what was stated before.

I have it downloaded on the console, no extra stuff.

I assume a public operating system build - how would i know?

If you have owned the console since new and never enrolled in the Xbox Insider preview (downloaded the Xbox Insider Hub and joined) then you will be on a public build. If it is a pre owned console, then download the Insider Hub app from the Xbox store and
open it. On the left hand side of the landing screen you will see settings, open that and go to manage devices. It will show there if it is enrolled and you can remove it if it is. They do recommend not using an extension (though most I would suspect, here
in the UK do)

Is your Xbox pre owned? I ask because your console maybe enrolled in the Xbox Insider Preview program. You can download the Xbox Insider Hub app and remove the console from the program.