Your basic walkie talkie won't work to well at WDW, you will need to buy the FSR (family Service Radio) that most electronic stores now sell. Best Buys, Radio Shack, Target, Walmart etc. The Motorola 250 model is the type that our family uses at WDE. It is about $60. It has multi channels to keep down on interference. It also has a large push to talk buttom in the middle of the front. Our boys have been able to work them since they were about 5 so your son probably won't have to much trouble.
Besides using radios just in case you get separated make sure your son understands who CM's are and how to identify them Name tags that have the white background....We have taught our kids to find a CM and tell them that they are unable to locate us, then understand that the CM will take them to a playroom to wait for us. That way we know where to start looking for them.