so worried

My son is 3 months old and I am so stressed and worried sick because his head is extremely flat in the back. I feel like I stress even laying him in the swing or down for bed because it'll make his head even worse. He takes naps in his swing daily mainly because he won't take naps in my arms he cries and fights it. I try to do tummy time as often as I can but he won't do it and I'm lucky if he can he will stay on his stomach for 5 minutes without fussing and basically burring his face into the floor. I don't know what to do anymore.