Loss of data

My son has an ID contract that gives him 2Gb of data per month.  He is nearly always on WiFi and keeps his data switched off, so 2Gb is way more than he needs. 

If he switches data on, just to browse for 15 mins on the bus - no streaming, no downloads - he loses data extremely quickly.  If he does this once or twice, he has used up the whole month's data. 

This is quite a recent problem - he used to use data on the school bus every day, and 2Gb was enough to last the month.

Is there any way I can see what his data has been used on?

Thanks

Marc