rolling onto tummy in sleep

My son is 5.5 months old and has been rolling really well back to tummy the past few weeks. Now he's rolling in his sleep like crazy. It's making me go crazy because he keeps rolling onto his tummy with his face straight down into the mattress! I can't sleep because I'm watching him in his bed all night long, checking him, moving him back onto his back. It's stressing me out! Should I just let him sleep on his tummy now? It still really freaks me out