If he goes to data usage, it will tell him exactly what apps/programs uses how much data.
I just looked at my usage data, and just a couple of you tube videos used the most data in the search period.
Here is a trick you might not know about.
Insert your phone in a foil lined bag like potato chip bag, and no sell signal can get to your phone. Tinfoil also works. Sort of like a Faraday cage if you know what I mean.
As soon as you take the phone out of the bag, you might have auto updating and messages data coming in.
I had about 15 items that also used data during that time frame.
Of course your friends can not get hold of you with phone in the bag, but then that might be good.
If your kids know about this, you can not track them either.
If you don't believe this, have your other phone call your phone in the bag. NO ring, no signal, no GPS, no data usage.
Works with any wireless device, wireless mics, wireless bugs, wireless tracking device , ankle bracelets, etc.
Why Does Aluminum Foil Block Cell Phone Signals? | Science - Opposing Views
You can try putting the MiFi in a silver bag, but the cord might be the antenna.