Tummy Sleeping!

DS just turned 7 months.  I just checked him and he's sleeping on his tummy, which I have never before seen him do (although he apparently does it at daycare).  He's rolling around a ton, etc., during the day.  That's fine now, right?  I put him in his crib on his back; he rolled onto his tummy himself.

I'm freaking out kind of, and fighting the urge to go roll him onto his back, which I know will wake him up and result in him sleeping in our bed (which we're trying to do less and less these days). 

 TIA!