My 4th and a half month old baby is sleeping on his back at night. Is there anything wrong with that?

A. There is nothing wrong with it if the child is sleeping on the back but the problem is the child is always going to sleep on the back the problem is that the head is going to become Flat because it’s very sensitive so that is why we suggested from the beginning itself to keep changing the sizes if you want to keep your child on the back only then start using a hollow pillow which would be very helpful