What is the best way to get myself out of this?

I'm just wondering what you think is the best way to get myself out of this situation.

I met a guy through mutual friends recently. We saw each other on a few different occasions and got on pretty well. Then a few days ago he asked me out. I was surprised as I hadn't really thought there was that sort of buzz going on between us, but I went along anyway. After we'd gone out two things were clear to me; firstly, while he's a nice guy I'm just not attracted to him, and secondly, he wants a serious relationship. He's just asked me if I want to go out again in a couple of days time, and I feel like I should put an end to it at this stage.

I've never been in this situation before (I'm 18 and have just had one long term relationship so far) so I'm not sure what to do. What is the most decent, least bitch-esque way of dealing with it? My friends responses range from 'you fooool, you should never have gone out with him the first time' to 'continue to go out with him but tell him you don't want anything serious at the moment'. I mean, should I just not return his call, or make a flimsy excuse as to why I can't go out? Should I tell him at this stage that I don't want a relationship at the moment at all (not quite true) or that I don't like him in that way (true but seems a bit mean?) Should I go out with him again and tell him at the end of the night? (On the plus side, that way I'd be telling him face-to-face, but on the minus side, that way I would have gone out with him a second time, which might be a bit unkind and leading-him-on-ish.) Or should I take the advice of one of my friends and continue to go out with him but strive to keep it light and non-serious?