Girls, hi. The baby is turning 9 soon, but he can't sit down yet. We …

Girls, hi. The baby is turning 9 soon, but he can't sit down yet. We saw a neurologist, she was very surprised that the baby crawls, walks near the support, but doesn't sit down himself, well even if we put him on for a minute to see how he holds his back, then he falls over. We went through a massage course, but there were no shifts. Who's kids sat late? At what age? What was the reason? Thanks.