Stiff neck?

Amber • 20 years old, mumma to my beautiful boy Oliver and expecting my second in Jan 2018 👶🏼
My 9 week old seems to be favouring leaning his head to one side and when I try to position it so it's straight he tenses up and cries as if it's hurting him! Does anyone know how I can get him to stretch it out so its not stuck on that one side?