Quoting Tiger Lily ♥:" Back, every time. I get so paranoid when I put him to sleep on his stomach. I've done it twice when ... [snip!] ... catch myself staying awake and staring at him..occasionally putting my hand on his back to make sure he's still breathing, lol."
If he can turn his head from side to side on his own, he should do fine with stomach sleeping. At about 3-4 weeks old, both of my kids stopped sleeping on their backs. They would wake up every time I laid them down unless I put them on their stomachs.
And as for the car seat thing, yes, if you were letting him sleep for ONE of his three hour periods in the car seat, then it would be similar to falling asleep on a long car ride. But to constantly let him sleep in the car seat for 3 days isn't good for him. That's all I was saying.
But try swaddling him and holding him tightly as you feed him his last bottle for the night and see if he'll hold still swaddled for you. If he's an escape artist even then like Autumn was for me, then try putting him to sleep on his stomach. He sleeps on your chest because he likes the way it feels to sleep on his stomach.