I just have to share this video from a local, Kansas City, horse therapy for kids camp. I've seen this therapy in action and it's amazing what they do for kids on the autism spectrum.
The horses are used to help children on the autism spectrum to recognize first the feelings of the horse and then their own feelings. The children also learn to follow directions so that they can do the things they want to do on the horse. All of the riding is done without stirrups and the children learn balance and co-ordination too. They are actually taught how to fall off safely.
Places like this need volunteers to help - often they need 2-3 adults working with each child at a time. It's a great learning opportunity for a teen who might be interested either in working with horses or with special needs kids.