I don't want to kiss him
Why don't ya want to share a smooch?
I can't put up with his farts and I feel like I'll vomit.
That's a pretty good reason not to want to kiss him, I'd say.  Just make sure you let him know!  You don't wanna just lead the poor guy on.
It's not that big of a deal. He'll get over it.
Well I think that he needs to be aware of how you are feeling. I think that if you don't, he might think that you are leading him on.