Good Lawd!
One day, you're a horrible mother if you lay your kid on his tummy. The next day, you're a horrible mother if you lay your kid on his back. "Rules" change all of the time. These experts come up with new policy and guidelines every day of the week.
If a baby is content on his tummy, and there are no soft articles (blankets, toys, stuffed animals, etc.) to get in the way of his breathing, then just leave him be.
You'll drive yourself crazy if you try to follow every single rule put out to the public.