my son's head started get flat on one side. but he had a thing where a muscle in his neck wasnt long enough so he could move his head to the other side. ( he was born with it).

When he's awake have more tummy time if he's able to keep his head up.. with my first we tried to avoid letting her sleep so much during the day so that she got on schedule at night and was only woken for feedings because I breastfed. The more you lay them down or let him sit in a bouncy chair where his head is down the less it will help the problem... I've always heard letting them lay too long everyday tends to cause the back of the head to become flat. Even bringing them on your chest during the day if they're not able to hold the head up just yet it creates a great time to bond and also have tummy time for them