Humping is due to a very normal instinct that all animals will have at some point, it's merely natural. Two of my male dogs (neutered) hump their stuffed "lover" dog, and that's fine with me...they've never tried to hump each other or humans, so it's fine. My female humps her donut bed
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I would just redirect him gently to doing something else and/or toward a designated stuffed "lover" dog like we have.