Here's a suggestion. Maybe as a parent, you might tell your son, you'll be monitoring his data usage and he has two options. Pay $10/GB of data he goes over OR if he can't or won't do that, take the phone away from him until he has data.
I get that you don't want to go to the Family Share plan, but to blame AT&T for your son not being able to accept limits or parental dictate says a lot........
