J.B.
S.~
My son was the same way. He didn't sit up until he was 8 months old and then he didn't do it well. He didn't crawl, not even army crawl until he was 2 days past his 1st b-day and didn't walk until he was 16 months old. He learned to scale a flight of stairs way before he could walk. Where he lacked in physical things, he excelled in others. He talked way early and has a huge vocabulary now and rolled over at 2 1/2 months. So don't sweat it. He's completely fine. My son is now 21 months old and very active!
Just my 2 cents. As long as your pedi isn't worried about it I wouldn't be.