Yes you are correct that you are playing with fire in keeping a male duck with female chickens. To be bluntly graphic anatomically speaking, male ducks have a corkscrew penis meant for actual insertion, while chicken sex does not involve penetration. Your drake could very well seriously injure the hens internally to the point they need to be euthanized. So separating him from the hens asap and getting him more female ducks if possible is definitely the best plan. Either that, or finding him a new home where he can live happily amongst other ducks. Sorry for your predator losses.