Probably a stupid question

My baby boy always sleeps with me at night. He always sleeps on his back. Which is how I lay him down to sleep because its the safest. Well for some reason he rolled on his belly to sleep. Should I try rolling him back on his back? Or will he be ok with me keeping an eye on him all night? I'm a very light sleeper and very in tune to him. He of course isn't surrounded by pillows or blankets.