Does he raise his head up, or does it seem stuck down like that. It could be wry or crook neck which is a neurolgical symptom of head injuries, diseases, and vitamin deficiency. As Suzie said, silkies with vaulted skulls are prone to head injuries, and they are also prone to wry neck. I agree with keeping him warm--put a thermometer on the floor and keep it at 80 degrees F, and keeping him quiet. Offer him fluids and electrolytes with a tiny bowl or dip his beak into it. Food isn't important for a day or so. Part his feathers just to make sure there aren't major injuries that are bleeding. It's not uncommon to find an injury later on. Get some poultry vitamins (many have electrolytes in them) and put them in his water. Chickens commonly will be in shock for a bit after an attack, so rest is important. Silkies are kind of fragile, so I hope he makes it.