I would start by downloading the My Verizon widget/app. It keeps track of data usage on the home screen to help prevent him from going over.
If he is adding the data plan in the middle of the cycle it'll be prorated so be careful.
Menu, settings, Wireless Network... should be something in there to the tune of mobile network or data network. You can turn it off there.
Turn wifi on and link it with home wifi. The phone will automatically switch to wifi when its at home.
Do major downloads over wifi.
The easiest way to explain hspa+ is 3.5G. Faster than 3g but slower than 4g LTE (which isnt real LTE, but the best America has to offer).
I got my son and my friend the same phone. Couldnt beat the price it was listed for online.