You really don’t need selenium, since it is easy to overdose. Just get some human vitamin E 400 IU to give once daily, and give a little cooked egg or sunflower see daily for selenium. Thiamine or vitamin B 1 is helpful in wry neck as well, since a deficiency of either B1 or E is a cause of wry neck. Thiamine is found in most chick vitamins, chopped liver, and in B Complex, which can be crushed and sprinkled on food.
Wry neck is just a neurological symptom, and can be seen in head injuries, vitamins deficiencies, and in certain diseases such as Mareks, fowl cholera, and other sebvere diseases. It is common in silkies and in chickens. They sometimes will walk backward or in circles. Make sure that he is getting plenty to eat and drink or you will have to help him.