Flat head?

Chelsey
My LO is 10.5 weeks old and since around week 5, I've noticed his head getting flatter in the back. We've brought it up with the pediatrician twice and each time he said not to worry about it right now, to see what it looks like at 4 or 6 months. But I can't help but worry about it! 
My son obviously sleeps on his back. We do tummy time during the day. He hates baby wearing. I switch his position in the crib. He won't turn his head while sleeping. I just don't know what else to try. I know helmets aren't the worst thing in the world, but I feel some mom guilt over this.
Is this the curse for a baby who sleeps relatively well?