J.C.
Hi A.,
Every child is different and boys are slower than girls so don't expect the same thing from him that you saw in your daughter. Not being able to sit up and not wanting to sit up are two different things and it would worry me if he could not sit up on his own. He has found that he can move from one place to another on his back so that is where he would rather be. Try doing a bit more tummy time and I bet you will see him start to crawl in about two more months.
Don't worry...he is completely normal! <grin>