Go to your local hobby shop and ask. I know they can run on a 2S lipo out of the box. Lipos are better, but you need a good charger that has a balancer with it. Check out hobbyking.com for batteries and chargers, cheapest prices I've ever found. The lhs will give you the right info then go find cheaper prices!
If you are just going to run it as is then get a couple 5000mah 2s Lipo's and a charger.
Hobbypartz.com has best bang for the buck combined with good shipping times.
You will need to get a couple packs of battery connectors as well.
The RTR Wraith is a blast but you are going to burn up that stock motor pretty quick. I would suggest gearing it down right away to help it last longer. Grab a 87 spur and 16 pinion. It will help your son enjoy it longer before it makes a smoke show with the stocker.
HobbyKing is similar, with great deals on batteries and chargers, as well. I've been running the Turnigy's Nano Techs from them in my comp rig for two years with awesome results and for my Wraith I run the Turnigy 2600 mah lipos and my Axial Honcho, I run two Turnigy 2200 mah lipos.
Like Harley says, a big mah 2s would be your best bet.
A forum community dedicated to custom off-road vehicle owners and enthusiasts. Come join the discussion about trail reports, builds, performance, modifications, classifieds, troubleshooting, fabrication, drivetrain, and more!