My little guy does it, he started just about at that age too. I firmly put him on his back and use a firm no. So now he doesn't do my arm he goes for my foot, I don't let him get away with it, its a dominance thing. You have to show him you are the leader of the pack! I too asked my vet to make sure and she said he is too young to be humping out of sexual pleasure HEEHe I hope this helps
