What helps with baby flat head? My little one is 4 mo old

I've increased tummy time but he sleeps a lot and just hates tummy time. If he hates tummy time how can I get him to sleep on his tummy?

My son hated tummy time and I would sit in the couch and put him tummy down on my legs and pat his back or bounce a little bit, even put some things on the floor for him to look at and sometimes he would even fall asleep like that.

Thank you so much ladies!