My sister has a 6 month old that still barley hold his head up and is no where near being able to sit up. It surprises me because my children were all sitting up by 4 months. Maybe they were just early bloomers!
Each child hits milestones at their own pace (that is from my wife). I am sure there is nothing wrong, or your doctor would have said something already. I bet he will sit up soon! Enjoy him being little, they grow so fast!
I agree with polamalu43. It really depends upon the kid. There can be a wide difference in siblings even. Unless the doctor is concerned, don't worry. The baby may just be a slow developer.
Don't worry about it. You would wish that he would still be on his back lying down when he is already running around and you have to chase him down. As the others have said, the baby might be developing at a slower pace which is not bad really.
