When can my baby start sitting down
All babies are so different and get to each milestone at different times. I have a friend who's baby knew how to stand against his play table unassisted at 5 months, but when he sat down he would topple over. Charlie is 5 months and is getting a stronger back and arms but it still unable to sit on his own.
The best thing is to just enjoy time with your baby. Keep your appointments and your doctor will know if there is reason for concern. Otherwise, do tummy time a few times a day, that will strengthen his back and neck muscles. Second, don't worry! He'll get there when he's ready :)