At the age of 6-9 months, your baby is able to respond to simple requests. By now he understands your language, so if you ask him if he wants dinner he would indicate yes or no according to his needs. In fact, he also understands some simple gestures from you, like ''come here,'' ''sit,'' etc.
What you can do
Talk to him more, which would help in developing his language and communication skills. Tell him different stories, different objects and ask him to respond. He will definitely respond back and this would enhance his overall developmental skills and make him learn new skills.
Beware of using proper words while communicating, as he will speak and talk in a similar manner. So, as of now enjoy those gestures of no and yes.