My oldest had a bad case of torticollis where he would only turn his head one way. Our doctor was very laid back about it, told us to just turn his head whe nhe was sleeping. That worked, and of course keeping him on his tummy or side. Also limit time in the carseat, bouncy, and swing because these all also put pressure on his head. When he's sleeping, turn his head to face one way, and for the next nap, turn it the other way. At night, turn his head when he falls asleep, and then turn it when you go to bed the other way. Of course he'll move around, but it will help.
Also, if you are bottle feeding, make sure you feed him from different 'sides'. Don't always hold him in the same arm, switch off so he gets used to turning his head both ways and using those muscles in both sides of his face. because believe it or not it does matter, if he only eats one way then the muscles on that side of his head, face, and neck will be much stronger and he'll favor that side.
Of course if you are breastfeeding you don't need to worry about taht since breastfed babies get that different side at each feeding.
ANYWAY, my oldest had a ridiculously flat head, so bad in fact his face was disproportioned. But after working with him and a little at home pt, turning his head and kepeing him off his back while awake, withing a couple months it was much better, and now at 4 1/2 his head is normal, nice shape, no sign of the flatness.