Until you get the bipap, prop him up. Getting a hospital bed made a big difference with mom and we could easily adjust it depending on what she was doing.
The neurologist told us when she was first diagnosed that it was normal for breathing to be 20% lower when lying flat than when sitting up. As breathing becomes harder, that 20% probably makes a big difference!
I have the mask with te nasal pillows, I dont have any trouble keeping my mouth closed at night so this works well for me. also mine has a humidifier which keeps the air from drying out my mouth and throat Rob