It's normal behavior. You might want to consider just letting it go. Sampson is 6-yo, neutered, still humps the big teddy bear every night. I never discouraged it as a puppy and he has never tried to hump another dog or someone's leg, so I just let him do his thing with teddy bear. Still makes me laugh every night. Oh, Daisy humps it too, but reminds me of those old bouncy ball toys we used to hop up an down on as kids. :-)     
				__________________  Marla, Sampson & Daisy     |