He may not have smiled back and had that look on his face because he wasn’t prepared for a smile. He is probably used to you not reacting to his staring.
To be sure that he is or isn’t interested, the next time he is staring, smile at him again.
If he smiles back, he may be interested and is now smiling back because he was prepared and was waiting for it.
If he still gives you that look and doesn’t smile, he may not be interested and may just be into playing with your mind and nothing more.