There are a lot of people on this thread that either have never owned or been around animals to any degree.A dog is very capable of doing this with or without training. 80 pound dog, 40 pound kid.Boars,stud horses,big dogs and bulls with a strong libido are capable of anything, and they can be quick about it.
That’s why women in their cycles should never be around a bull or horse. There’s an interest there. BTW, I am a dog breeder.
I suspect that it was a dominance thing.
The kid had just messed himself. Dogs mark territory with their droppings and and urine and they will mount a male if you’ll forgive me, to end a pissing contest.
Its unfortunate for this kid that his parents are morons