How can I let this guy who is flirting with me know that I’m not interested without sounding harsh?

Sit down with him somewhere, where you can talk in private. It's important you do this face to face, even though that's difficult. Calmly tell him something like "look, you're a cool guy and I like that we became friends, but I'm not looking for more. I hope we can still hang out as friends, that would be great". Something among those lines would do. He might not like that it's not mutual, but it's important to be honest. This is one of the nicest ways to tell him you're not feeling the same for him. It's not going to make you sound harsh.

Honestly just being honest with him is probably the thing he would want you do because giving him small hints will leave him confused. You may feel bad for hurting his feelings but its better than making a lie. I mean i would want them to be if i was in his position. Just simply saying that and not letting it become awkward for you or him.

Something along the lines of this, from 'The gift of fear':
"No matter what you may have assumed till now, and no matter for what reason you assumed it, I have no romantic interest in you whatsoever. I am certain I never will. I expect that knowing this, you’ll put your attention elsewhere, which I understand, because that’s what I intend to do.

There is only one appropriate reaction to this: acceptance. However the man communicates it, the basic concept would ideally be: “I hear you, I understand, and while I am disappointed, I will certainly respect your decision.”

No matter what you say, it will probably sound harsh to him because he's being rejected. That being said, there is a right way to talk to people, and a wrong way.
I would ask to speak with him in a public place with lots of people around and tell him in a straightforward manner: I am not interested you at all. Do not contact me any further unless you want to be just friends and nothing more