Child protection

You can request a feature update here - Product Feedback - Apple

Currently there is not way to restrict it in settings. Ask your carrier to turn that feature off on their cellular plan.

Who is your carrier that they are telling you they cannot disable Personal Hotspot? The ability to activate Personal Hotspot on your device is controlled by the carrier, and they should be able to disable it on your son's phone, based on the individual account. I suggest you contact them again about disabling Hotspot, since they are the only one that can activate it for an individual device, they can turn it off.

As pointed out, the ability to do Personal Hotspot or not is actually a carrier controlled feature. I'm not sure if carriers typically can (or will) turn that off upon request. (Usually the issue people run in to is that people want to turn it on and the carrier blanket-doesn't-support it).

Now I don't think this will work unless it's been changed in iOS 12, but it's certainly worth a try to do the following:

Since you can use parental controls to "Don't allow changes" for "cellular data changes" and Personal Hotspot would appear to be grouped with those settings, I do think this would be worth a try.

https://support.apple.com/en-us/HT201299

https://support.apple.com/en-us/HT201304#allow-changes

I will confess though, my initial reaction in reading your post was "wow, smart kid!"

None that are not parental.