While he might be missing some parts, the hormones are still there. It takes several weeks for the hormones to flush completely out of his system. Another possibility is that he is simply uncomfortable. He might find humping to be weirdly comforting. He's a pup and has no inhibitions like we do. If he tries to hump, say no, pick him up and remove him from your leg. If he insists, walk away. He can't hump you when you're walking.
__________________ Littlest JakJak 
We miss you Kaji 
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