You can add Family Base for $5 a month which will allow you to control when he uses it and how much. I've read a few posts where it didn't work or lost some features with certain phones so I would poke around on the forums about it so you can make sure he has a compatible phone.
Some phones also allow you to limit the data right inside the device; S5 for instance. However it doesn't have a password so he could easily just go in and turn off the limit.
The suggestions made thus far are your best bets. Trying Family Base or doing it the old fashion way and taking his phone away from him until he can prove that he can respect the sharing aspect of the data.