I know that the Neverland Club has free activities for kids in the afternoons. This is located at the Polynesian - not sure if you have to be a guest there or not to enjoy this.
Depending how old your son is, they could also rent watermice from the marina at the GF or Polynesian. They could go on a fishing excursion. They could go to Fort Wilderness and explore. They could take the Disney Family Magic Tour.
Hope some of these help!