Has he been like this since hatch? Could be a deformity, if it's physically twisted and has been like this since hatch. Otherwise, wry neck, Marek's, or a neurological issue could be the cause.
It's probably wry neck (torticolis, crook neck.) It can be a symptoms of head injury, vitamin E deficiency, Mareks disease, and many others. Vitamin E and selenium are usually given for treatment daily. Selenium is found in egg, tuna, sunflower seeds and many other foods. Poultry Nutridrench contains both selenium and E, and dosage is 2-3 ml daily by mouth or in food.