JD5,
There is a lot to undertake being a newby and coming into the racing world without much knowledge of how to do so......... but the people are all great and will be very friendly and helpful to get you going in the right direction. First order of business is your son and his bike. The KTM 50 SX Pro Sr is a "Peewee" class bike and really set up for kids that are 7 - 8 years old and with an 11 year old............. he's to old to legally be able to race that bike against the the 7 - 8 year olds. The next step up is the Beginner 65 class where he can be 7 - 11 years old and race up to a 65cc motorcycle (and I believe he can race his 50 in there too, but he'll be against kids his age). The beginner 65 class usually runs on peewee tracks as these kids are just getting their feet wet into racing and learning how to ride, much like your kid. The regular 65 class runs on the big track and a big step up from the beginner class, but something for your son to watch.
Another thing to keep in mind is when the age groups are effective for youth racers. Your childs age on Jan 1, 2011 is his age for the entire 2011 season. (Example, of your kid turned 12 on Jan 5, 2011........... he's considered 11 for this entire racing season and doesn't have to be in the 12 -13 year old class until next year).
CRA also has the "trailbike" class which is great for kids that are older just getting started. It's for older kids riding less competitive models of bikes (not 65cc or 85cc two strokes) to get used to racing on the Peewee tracks as well. Your kid might want to get started here with a Honda CRF 70 or CRF80, or Yamaha TTR110, or Kawasaki KLX110, or Suzuki DRZ 110 in this class. It will be similar to the beginner 65 but with less "racy" motorcycles.
Just some things to think about as you get ready to get your feet wet in racing.