What phone does he have?
How much data is he using?
If you look in his phone under cellular data, is there a particular app, or apps that are using a lot of data?
I wrote this for the community. It's the result of my own on again, off again struggle to understand and control data consumption. It might have helpful information.
One of my boys was using too much data (you tube every chance he got). I showed him how to monitor and control his data and told him once he hit his limit, I would turn data off until the new billing period. Then I followed through as soon as he went over. It was a lesson that didn't need repeating.