Don't like him as much as I thought I did?

So I work at this summer camp and I started to go out with this guy a couple of weeks ago(he also works at this camp). uhm. His sister(Alejandra) works with me and my brother(jarrod) works with him. My brother goes out with his sister so we're those two couples that do everything together.

Anyways; When we first started going out I thought I was crazy about him and now-well not so much. Last night we all went on a date and we kissed for the first time.(me and this guy;Alejandra and Jarrod) He held me in his arms and whispered that he was going to keep me untill he dies. :/ and thats when i freaked out. I didnt say anything but inside I was like- Dude. I'm 13. Seriously?

Long Story Short: I don't like him as much as i thought i did. He's already thinking about marriage and i think i could find someone better. He has a short temper so i don't want him to flip out at me in front of the kids at camp if I brake up with him.

Help?!