It doesn't have a cash system per-sae.
If he plays on Xbox just get him Microsoft points. If he plays on Ps3, just get him a PSN card.
They're similar to Ultimate Game Cards and whatever else uses cards that you can buy from a 7/11/Walmart/etc.
Edit: I also know that they sell DLC cards at GameStop for specific things related to CoD and other games. Just be sure you know which CoD he plays. They don't all count for the same title.