How should I go about this?

My son has started trying to sleep directly on his face. Literally, face shoved into his mattress. It scares me, because he will fall asleep on his back or side, but I'll wake up at 3am and see him sleeping on his face so I'll run in and reposition him. I had to reposition him 5 times last night..and it made it really hard to sleep because I was scared. It would be one thing if he turned his head to the side..but he doesn't do that.. I'm not sure if I am overreacting & should just let him or if I should continue moving him as I see it. I don't know what to do 😫 I read that people have bought an owlet for this reason..but he is 9 months so I'm not sure if that would be a waste of money at this point. Has anyone else gone through this?