Cubby had the same problem. My vet gave him medication for it. They put him on ovaban, which he acted pretty druged and depressed, so I took him off of the pills. I would rather deal with a humping cat than have a depressed cat. You can take him back to the vet and ask, they told me he couldn't figure out when it was play time and when he wanted to hump which is why they put him on pills. I would call other vets and ask around. Or at least that's what I did. Hope this was of some help to you.

Katie