Replied to the wrong person...let's try again.
Switch phones with him and see if the problem follows whatever phone he uses. If it does, you know the issue is his usage. If YOU now have a big data problem, then you know it's, FOR SURE, the phone.
Here are the measures I have had to take to control my 5C data usage. I posted this almost 2 years ago, but the data points still remain valid today.
You also need to turn off location tracking services: Settings => Privacy => Location Services => scroll down to System services => scroll down to Frequent Locations => Clear them and turn this function OFF. The factory default is set to monitor and track your where-abouts, and yes it uses cellular data to accomplish this. This is built into iOS itself, and not associated with any app.
The best thing is to stay on wifi. But, yes there are things one can do to minimize data usage while on cellular. No big deal. That includes turning everything off in System Services at the bottom of location services. That stuff is not necessary. I wouldn't turn all of location services off though. Need some things like weather and Find My iPhone. It's mostly what is done by the user.