Hi Myra,
Good job on mustering the courage to talk to him.
He most likely only says hi and doesn’t try to talk to you because he is still that shy guy and he can’t help his nature.
One conversation with you in his office is unfortunately not going to change that.
He is worried that you may be expecting him to return the favor in initiating a conversation with you and this alone may make him so uncomfortable that he is now not even trying to talk to you.