Hi Mary,
The
Tri-Circle-D Ranch at Disney's Fort Wilderness Resort and Campground is great. My daughter took a pony ride there years ago, and she still remembers that her pony was named Nemo! You take the same internal Fort Wilderness Bus to get to the Tri-Circle-D Ranch as you do to get to Mickey's Backyard BBQ.
At Tri-Circle-D Ranch, you can simply explore. You may see some of the horses who work on Main Street in Magic Kingdom Park taking a break! For a small fee which does not require reservations, you can take a wagon ride. These are on a first-come, first-served basis. Currently they depart at 7:00 p.m. and 9:00 p.m., so this would be something to do after your dinner. We went on a wagon ride with my family once, and it was a relaxing way to end the evening. Pony rides are sometimes offered for a small fee at the Tri-Circle-D Ranch, with no need for reservations.
Options which cost a little more money and do require reservations include going on a horseback trail ride or a private carriage ride. Please be aware that for the horseback ride, children must be at least 9 years old and 48 inches tall. My family has done the private carriage ride, which was nice because we talked to our guide about the horses and the campground. You can reserve private carriage rides and horseback trail rides by calling (407) WDW-PLAY (407-939-7529).
I do think it's worth it to visit the Tri-Circle-D Ranch, especially because your son loves horses. Please be sure to check daily operating hours before you go - sometimes it closes early. If you go a little early and have time between visiting the horses and Mickey's Backyard BBQ, there is a great playground quite nearby where your son can play until it's time to eat.
Have a great time planning and do not hesitate to ask if you have any further questions, Mary!