It sounds like you are an attentive, hands on, dad. Way to go. I find it tough to be a good parent with the PCS. I would treat head butting like I treat the stove or other hot or really dangerous things. I firmly say (sometimes shout if I'm a distance away) "no" or "hot". I use a tone of voice I don't use for anything else except the really, really dangerous and generally my three kids have picked up that this is something really really forbidden. When my kids were toddlers I did not have PCS (thank god), so I didn't use the tone for headbutting so I don't know how it will work for you. Good luck. I do remember those headbutting days.