That is great your offering this boy a chance to come out and be with the horses, I was a foster parent for 8 years and the horses were very therapeutic for my older kids in care.
Explain the rules, let him know the consequences for not following the rules. Children his age are able to understand rules and consequences very well, keep them simple, black and white. Then give him a curry and a brush and teach him how to brush the horse Show him the hooves, teeth if your horses are good about getting hand fed let him give them a little treat, if not let him hold a bucket with a little treat in it.
I would also have him help out with some chores maybe cleaning water buckets, cleaning a stall or two. Kids this age generally have shorter attention spans and like to change activities after about 15 minutes.
I would definitely find a way for him to get a lead line ride on the horse, every kids fantasy of horses is riding them, even it is only for a short time on a lead line. You will build him up and give him confidence, it would mean the world to him I am sure. Have fun, and take lots of pictures that he can have to keep.