J.S.
My daughter is a little over 8 months now and she's no where near crawling yet. She rolls from tummy to back when she feels like it, but not back to tummy, doesn't get up on hands and knees, but has started climbing on me when she's on my lap. I wouldn't worry about where you son is at right now. That sounds normal. If you're really worried, you can always ask his pediatrician if he/she is concerned about his development. I have friends' babies who are a little older (9 or 10 mos) and already crawling or walking (ah!) and another friend's baby is 8.5 mo and no yet sitting up. Like you said, all babies are different.