I’d consider his diet as the culprit. Also consider a homeopathic practitioner to help you suss out what might be causing his symptoms. You need to begin thinking outside the box. Also from a Google search I found these natural suggestions listed below. I hope you both find peace and a happy gut.
From Google: To naturally relieve nausea, you can try:
Eating bland foods: Opt for crackers, toast, plain rice, bananas, or applesauce, which are easy on the stomach; eat small meals frequently throughout the day.
Drinking fluids: Sip on clear liquids like water, broth, or ginger ale, and consider cold drinks
Ginger: Consume ginger tea, ginger candies, or chew on fresh ginger as it has anti-nausea properties
Peppermint: Inhale peppermint essential oil, suck on peppermint candies, or drink peppermint tea
Acupressure: Apply pressure to the P6 point on your wrist, which is located between the wrist crease and the elbow
Aromatherapy: Inhale calming scents like lavender or lemon
Fresh air: Spend time outside to get fresh air
Deep breathing: Practice deep breaths to relax your stomach muscles
Lemon: Smell a lemon slice or drink lemon water
Hydration: Drink plenty of water to avoid dehydration