6month sleeping face down

My 6 month old boy has beginning to flip over and sleep on his stomach. Last night I kept getting up every hour to check on him in his crib and he would be face down! I turned on the lights and looked at him and seriously don't know how he is able to breathe!! I would flip him back over on his back but I would go back an hour later and he would be sleeping face down again. Does anyone have any advice? We don't co sleep due to not enough room in our bed plus my husband gets up at 3:30 am for work and has to be in bed by 8pm.