Hi Amanda,
We were in the same predicament earlier this spring during our stay at Disney's Fort Wilderness Resort. My 8-year-old daughter was too old for a pony ride, but not yet old enough for a trail ride. There many other recreation experiences at Disney's Fort Wilderness that your son might enjoy.
You can always explore
Tri-Circle-D Ranch with your son. He can walk through the stables and see the horses from Main Street U.S.A. You could take a wagon ride or carriage ride with him. The horse-drawn wagon rides with a driver typically depart twice a day in the evening, weather permitting, and are first-come first-served. You will ride through Disney's Fort Wilderness Resort with many other guests. The carriage rides are more private experience where you will ride a horse-drawn carriage with a driver. We have done this experience as well, and the drivers are knowledgeable and will be able to engage in discussion just with your son if he has questions. His little sister could also do these experiences; however, you could make them special just for him.
For a non-animal option, your son might enjoy the
Fort Wilderness Archery Experience. Guests do have to be 7 years old to do this, so it would be something special that a "big kid" could to but not his little sister. During this experience, he will receive training in how to use a compound bow and then use it to practice shooting at bull's eyes. This experience is limited in the number of guests who can participate in each group and is offered at specific times, so do check with the resort when you check in for times and availability.
I hope that these thoughts are helpful for you, Amanda. Have a great time planning your vacation, and please do not hesitate to ask if you have any further questions.