The reason the positional "treatment" may work is because it beats gravity. When we sleep on our back, all that material (tongue, muscles, tissues, fat, etc) collapses because the weight of itself is more than the material itself can handle. So by sitting up or being on the side/stomach, we help take the weight of that material mostly away. Some snoring and even collapsing of airway may still happen, but it should happen less.
In the meantime, tell him to take control of his own condition. Research. YouTube. Search here. Learn all he can about it so that he can face it without fear. Respect, but not fear. Learn even more so he can understand what machine he may need so he can get the best. If he has mixed apnea (you said he was diagnosed with both obstructive and central?), then it is even more important he get a data capable machine.
This is his condition. His life. His fears. Face it, beat it. And we are here to help do that.
Take a deep breath and count to zen.