Some babies do experience flat head. I’ve been using flat head pillow and rubbing her head clockwise since birth and it works pretty well tho. Her head is very round as mentioned by people around her 😅.
And no, flat head will not affect his brain or function. It’s more obvious because they don’t have much hair, once their hair starts growing, you can’t really see the flat head.
Anyways I used pillow since there is always people with her. But if baby is sleeping alone, best to do without the pillow. Try helping him to switch sides when he sleeps.