When can we start putting a pillow behind our LOs heads when they sleep? Are there any pillows that can help prevent flat head?

I introduced pillow to my boy when he is 3 months old, when he is able to lift his head up and do tummy time. He seems to sleep better with pillow. I personally think that it will be safer to introduce pillow when baby can lift his head up during tummy time to reduce the risk of suffocation. At 3 months old, baby can turn his head left and right so flat head shouldn't be a concern.