flat head won't go away😩

Kayley • 24 years old, I have a little boy who’s 4 and I’m TTC my second baby!! 👶🏻
My son is 4 months and has slight flat head at the one side, he has lots of tummy time and is barely on his head apart from when sleeping. How can I get it to go away? He's had it for over a month now😩