i might be able to help you.
rhys is 3 and he has humped things since about 8 months, it happens when he is tired and it seems quite comforting to him. when people come over they get freaked out but for us its a serious issue.
in the past few days i have spoken to people who work with autistic children and they think that along with the humping, speach difficultys and OCD that rhys might be borderline autistic
humping/bouncing is one of the typical signs for somebody with autism as it helps to sooth them. we have an appointment booked to take this further but ive always had my suspisions that rhys is autistic.