Why is the right side of my baby's head flat? he prefers to sleep on that side. the right side of his forehead sticks out a little too.

Recheck with doctor: Positional plagiocephaly happens when babies sleep in the same head position too much of the time. A baby's skull is "soft" because the bones are still growing and are not connected together yet. His skull gets flat where it is constantly being pressed. One can try putting him down with the pointy side of the back of the head resting on the bed. Pediatrician, and sometimes neurosurgeon, can help.

Water balloon: If you remember, a water balloon is always flat on side it lays on. A baby's skull has "floating" bones that push away from each other as they grow larger until they are ready to knit/fuse.This makes it like a water balloon because gravity treats it the same. Your doc needs to examine the baby. Position changes and therapy can help.The issue tends to fade when babies sit up more/sleep less.