My son scratched off his gift card code

See this support article:

http://support.apple.com/kb/TS1292

If you can't get to work after perusing that page, contact the iTunes Store Support; instructions are at the bottom of that article.

Regards.

Forum Tip: Since you're new here, you've probably not discovered the Search feature available on every Communities page, but next time, it might save you time (and everyone else from having to answer the same question multiple times) if you search a couple of ways for a topic, both in the relevant forums and in the Apple Knowledge Base, before you post a question.