This is from allearsnet in regards to the waiver/age/weight restrictions for the Trail Rides at Fort Wilderness.
At check-in, we each had to sign a waiver agreeing that we would not hold Disney responsible if anything happened during the trail ride -- we were also instructed to indicate our age, weight, and riding experience at the bottom of the waiver. Riders must be at least 9 years old and weigh no more than 250 pounds (113 kg). Double riding is not permitted. Riding experience is irrelevant, however -- anyone who meets the age and weight restrictions may participate.
Yes it's horse back riding, not drinking but don't forget that horses are living breathing creatures that have a mind of their own. I have spent the past 20 years of my life around horses and I know how easily even the most laid back of horses can hurt you. As to the argument that children younger than 9 are riding in your area, I'm sure that's true. The lesson barns in my area will start children at age 7 but they are in private lessons in an enclosed arena on a pony or they've grown up with horses since they were born.
I've done the trail rides at Fort Wilderness and while you do not go any faster than a walk, everyone is in control of their own horse, there will not be someone to lead your child's horse. You are also riding a trail through the forest and back to the corral. The trip takes 45 minutes approximately in total. Why not wait one more year until he is of age and maybe try some trail rides in your local area to make sure that he is comfortable on his own on a horse.