Baby is almost four months old and the left side of his head is pretty flat 😩; in comparison to the right

He loves turning his head to the left, when he sleeps or even when in bouncer or car seat.. I’ve wedged things under his head to keep him from laying on it, and he doesn’t like it and will always adjust his head to make it turn the way he wants lol stubborn little man 🤔 I try to do tummy time frequently to keep him off of it, but his two hour naps , don’t help lol.. I do hold him for his morning nap, so at least he’s not on it, then ... I’m afraid it’s going to stay flat 😐