There's definately no need to worry. The habit of comparing to other kids is so hard to break, but it's totally natural. I used to do it all the time with my son, and it worried me to no end when he wouldn't roll over or crawl. He was sitting up at 5 months, but he just wouldn't move when he was on the floor. And once he learned to sit up, there was nothing better! He didn't crawl until he was 7 months old! The day he crawled for the first time it was a total shock to everyone! (I'm just glad I was there to see it) I think he was just saving his energy for the fun stuff. From the first day he crawled, he only used crawling to get him closer to something he could pull up on. He walked at 9 months, and never crawled again! (and he never mastered rolling)
As everyone else already said, every baby's different. You just can't say it enough! That one little, seemingly insignificant thought has to be every mother's first thought when they get sucked into the comparison trap. (even the one's who are internally gloating over their child's apparent advancement. lol...I do that a lot) Don't worry about your son, he sounds like he's moving along just fine. My son is 2 today, and I'm wishing for those days when I could just plop him on the floor, and know he was safe!