What Cat Herder said!! ^^^^
I also think it may not be a case of he WON'T sleep on his back but more of he doesn't know HOW.
If he is given the choice of sleeping in a swing or being held, he was never taught how to sleep properly.
He is very young so he doesn't know. He is only doing what he was taught so far...sleeping in a swing. Mom needs to teach him to sleep on his back.