I don't think you're being very fair. Of course you could promise him fidelity! I get horrendously drunk sometimes and whenever I get into any kind of cheating situation there is still something in the back of my head that says "STOP!" However drunk people are I don't see how it's possible to just totally not realise, unless you're unconscious or something, but surely drinking THAT much isn't necessary. You can be off your face, have a great time but not compromise a relationship.
I went to Zante last summer with a load of girls after exams, and me and 1 other girl were in relationships. Neither of us cheated. There were occasions where holiday reps would be getting us to play drinking games involving pulling random men, but I just said "sorry I reeeally can't!" and stepped back. Normally I would be up for joining in but I just wouldn't do that to my boyfriend. Besides, it was quite funny watching my friends down cocktails and snog complete mingers!
I think you need to decide how you feel about this guy. His feelings are totally understandable, especially given his past and the fact that you can't reassure him that nothing will happen. If you decide he is worth sacrificing a tiny (and it reallly is tiny) part of your holiday for, i.e. the random pulling bit, then you will earn his trust. However, I reckon if you decide you can't then that shows you're not up for a serious relationship. Which is fair enough, but he needs to know!
Sorry about the ramblingness, I'm not feeling that concise this evening!xxx