Mine does that too, most frustrating when he wakes himself up rolling. My pedi said that it's not really an issue if he can roll back to front himself.
Both our girls roll over immediately. I've tried to explain to them that their blanket sleepers even say "back is best" but they just don't listen to me.
DD loves her tummy, she's runs her whole crib on her belly. It makes me a bit nervous its normal. I check her during the night, when I had my first some time ago it was said that the "side" was best putting rolled blankets against his back.
Re: Any LOs now insist on tummy sleeping??