What are gigabyte on an iphone/?

I bought my son a Iphone 5 for Christmas. My plan is for 10 gb. He has used 6.5gb in 2 weeks. What could he be doing on his phone to use so much data?

He could be watching lots of videos and social networking, or he could be using the connection to setup a "personal hotspot" - sharing the connection via Wi-Fi with other devices.

Well could be anything. Movies are the most common for eating into download quota.
Whatever it is you will have to keep a check otherwise you could be in for a big bill at the end of the month