Hey.
First, I don't know how you got hooked up, but to prevent this from happening in the future, you might want to escape from the things that brought you together in the first place. This will stop this from happening in the future with another person. Find people to be with on your time.
Do you think you can go straight after going to third base with a guy twice? This is going to be hard in my opinion because after the first time, you would of known whether or not you really liked it. Because there was a second time and it happened with the same guy, tells me that you liked it. And it might be you just don't want to stay with him. Not all guys in general. You might be sacrificing all guys because of one. Why the change though? Peer pressure? Family pressure? Don't try to hide your feelings because people seem to be pushing it that way. Be your own. Your happiness is what matters.
It is hard though to break it to someone that you only want to be friends. But some simple ways are here: Write a letter, call him, text him, and face-to-face. I put face-to-face on here because this could give him real facial expression and drive it home to this guy. He will see in your eyes that you might have made a mistake with him. He might even understand fully. I listen and respect someone when that tell me things looking in my eye because it tells me they have good reasons for telling me what they're saying. And it shows they have that respect for me to confront me directly. I may not like what they say right at that moment, but I understand later. Next is writing a letter. This could be a simple solution if you just don't get the nerve up. It can be done to make sure nothing is left out. Don't just tell this guy to go away. Explain to him why. Say all of your feelings. A phone call can be sincere too. He can hear it in your voice. A text, in my opinion, is last resort. Only do this if the others you just can't bring yourself to doing. Text letters can be taken way out of context because they're short and simple. This isn't what you're aiming for.
And he might be hurt, he might not. He might even feel the same way. You said he wants the relationship. Did he tell you this, or are you assuming? There is such thing as good friends. But if this doesn't work, then don't hesitate to slowly start breaking away. Miss one phone call. Then miss another one. And then don't show up. He'll get the picture. This isn't a slam on fest. Progression is the key. He'll finally get the picture and lose interest or those strong feelings if he has them.
But don't be scared. He is a big boy. He is gonna get much harder in the future. This isn't really that bad for him. So many fish are in the sea. His will come. And yours will too.
Take care.
~jamesish~