Faolan would not sleep on his back at all until he was 6 months old and even now he only sleeps on his back for naps.
Tummy sleeping may just be a preference but you may also want to check for reflux or silent reflux. Faolan has it bad though he didn't turn into a total nightmare until about 2 months. Refusing to sleep on his back was our first clue.
H&T Mama to Faolan
6/7/12 & Malachy