Well it could be he is unsure of himself I would suggest asking him to write down all of his likes in a fantasy or a few fantasies he has and you do the same then you compare and find middle ground maybe by working together you can guide him and he will feel like he isn't just shooting in the dark. I also know there is a book The New Topping Book, I have not read it though I have read its sister The New Bottoming Book and found it very helpful. I though don't know your husband and how open to any of this he will be he may just feel unsure about whether when you are saying "no don't" you mean no don't or please yes.