There are a couple things to consider, are you sure he's getting enough? If he is then this is just a soothing measure and for your sake (and the sake of your nipples) you shouldn't let him suck longer than 30 minutes on each breast. The other thing is, he could have a stomach ache, babies will suck if they have an upset stomach, it could be caused by gas pains. I'd make sure that he's getting enough, and that its not a stomach ache.