What’s his weight? If he is overweight, ask him to reduce weight. Quit all forms of addiction, especially alcohol and smoking. For temporary relief, you can try elevate his head end by using an extra pillow. You may also try and use a body pillow. Ask him to lie on his side while sleeping instead of lying on his back.
Health Tips
Be cautious about using elevated pillow in the long term, as it could lead to back and neck pain.