my 6month old son still isnt sitting up help?!?

alaska • mommy of an amazing little boy and another amazing little baby on the way! happily engaged to the love of my life!
what can i do yo help him learn to sit up hes not doing anything any of tge other babies are doing he rolls over and thats it i give him lots of tummy time i help him to sit up and he just falls over still im really worried about him picture for attention