(1) Does he not know?
(2) Doesn't he know?
I don't usually see and hear questions formed in the first style. I was even surprised to know that it is grammatically correct. And actually it is the most "proper" way to form an interrogative sentence. Ngram shows that the first style was much more popular before than now.
What are your suggestions on choosing between these styles? And I will probably add the third one.
(3) He does not know?
The answers have explained how to form a question, but have not actually provided an account on the usage of the first two forms. Especially I'm interested in the first form: does it sound formal, is it used in speech or only in writing, et cetera.