"The hero would never say that!"
Ever heard that? Ever said that?
While I'll agree that there's always somebody who's willing to say something so dumb that it can't possibly be real--oh, wait, am I talking about reality or fiction?
I recall a time long ago when I was under a lot of stress at work. The stress came out one night through a rather disturbing nightmare about my ex-wife returning to the house and shooting me, and then my girlfriend. Now, the ex-wife in question would never actually do that (at least, I don't think she would) but it freaked me out nonetheless. And then, inexplicably, I told my girlfriend of the dream.
Hey, I was stressed out, remember?
No surprise that she broke up with me very closely to the revelation of the dream, is it?
Dumb, dumb, dumb, and I knew it was at the time. Still, I was stressed, and often when we're stressed our mouth runs away with stuff the brain hasn't okayed yet.
Next time you put your protagonist into a stressful environment, which should probably encompass the majority of your plot, go ahead and play around with letting him say abnormally stupid stuff. It's--normal.