It's not funny, but I got a mental picture of milo humping what I am assuming is a crawling baby and I had to laugh!
sorry! Anyways, just firmly tell him no, and take him to a "time out" place. I have a neutered 4 year old and he is the only one out of 3 boys who humps. But, he knows he is only allowed to have "special time" with his blue loofa dog. Good luck!