Most Helpful Girl
You know how to do this, you just feel guilty about doing it.
He's a nice guy. You're a nice girl. Tell him that you've had a good time but you're just not feeling that there is anything more in this for you. In other words, just tell him the truth. You're only 20. You shouldn't be expecting to meet the love of your life just yet. And nor should he. Do both of you a favour and free you and him up to meet someone that you can both have a stronger, happier relationship with