When a dog mounts a child or even an adult it is most often a demonstration of social dominance, not an actual attempt to breed. Dogs commonly do this to each other as well. It is a non-aggressive way for the dog to show that he is above whomever it is he/she is doing it to. Sometimes it is even a female dog doing the mounting.
It is possible for this bizarre rape to have happened, but I do have doubts until I see proof. This is because even when dogs breed, both male and female must position themselves properly. Plus, the poor child would have had to have no clothes on from at least the waist down.
If this did actually occur and it was a true sexual act on the part of the dog, most likely the dog smelled something on the boy that would have sparked him to attempt to breed. Dogs are go by smell when breeding, first and foremost.
I am sorry if this sounds just awful, but I am just trying to explain what I can for a better understanding.
Lion