Actually, that is very typical of mink attacking young fowl/poultry. they decapitate and eat the head and lick up the blood as it spurts from the arteries. The can squeeze their bodies through an opening less than an inch across. Weasles will do the same, they are a little bigger though.

I raised all kinds of poultry, waterfowl , and game birds for years and saw that a number of times.

From your description of the incident, it sounds like what happened. But I wasn't there, obviously.

Human or animal, it is still a tough thing for your son and you. So sorry.