I Fear That My Boyfriend Is Jerking Off While We

I fear that my boyfriend is jerking off while we sleep -- it's been going on for a while now and because of his breathing, twitching and sounds he makes -- his body tenses -- all as if he was have an orgasim. It keeps me up all night and we are having serious relationship problems because of it. I don't get any sleep and while he says he is not -- I just don't believe him and he thinks I'm crazy.