How to avoid flat head?

My son would prefer to be on his back, which is the opposite of how he used to be 🙄 he lays in his boppy a lot, sometimes his swing and then at night he’s laying from 10pm-8am ish. He’s starting to lose hair from laying on his back which I know is normal but how do I avoid his head getting flat? He is almost 8 weeks old. I don’t remember having this concern with my first. But now I have a toddler to entertain so my son is set down a lot more than I set down my first.