I had a spayed female Schipperke who would hump our cat occasionally. I also knew a female American Eskimo dog who humped people's legs. None of my male dogs have shown this behavior.... ever. Oliver was neutered at 5 months and Ziggy was over 2 years when he was finally neutered.
It will stop, and I think that if you make a big deal out of it and draw attention to it he could possibly do it more. Puppies are like children and will continue behaviors that get them attention. A quick "No" and redirect him to another activity or toy should be all that you need to do to get him to stop.
__________________ Melissa and Belle  |