Baby turning

My baby boy is 5 months old and has started to turn since few weeks ago. But nowadays, he likes to turn on his left side and eventually end up facing down. As it can happen a few times at night, we are afraid that he will experience numbness on his left arm or his breathing is blocked by the mattress underneath.. I tried to place his bean pillow underneath his left arm to prevent him from turning over but he easily overcame it these couple of nights. Any mummies and daddies experience the same with your LO? What else should I do to stop him from turning over in the middle of the night?