Some babies don't burp much at all, but if he is passing gas then he is getting rid of the excess wind that way I guess. When you do burp him and he brings nothing up, do you feel that he does have a pain there but just can't let go of it? In my experience, I have found that if they are too tense to just let it go, then it can make the situation worse sometimes. If that was the case, I would just leave, them, maybe tickle their feet or something that would relax them and more often than not as soon as I sat them up, they would burp.