Chris.
I don't think there's any difference in online price and buying them at the park but it might be faster online. Your son will enjoy disney world. If you go to EPCOT it's best to go to the world showcase first then backtrack to the front in the afternoon. If you want to eat at one of the EPCOT countries you'll need to get a reservation as soon as you enter the park. The food on International Drive is not that good. You may want to drive out of the park to west 1-92 or Sandlake Blvd to eat. There's a Beef O'Bradys on 1-92 west of I-4 that caters to the Brits for Soccer. Very little Foster's for you Aussie's. Have fun.
JJ