You say he gets nauseous when he eats and gets headaches. A sluggish digestion from too little stomach acid can do this to you. Here is something harmless he can try. Have him take a spoon of apple cider vinegar just before a meal. If the symptoms go away you have your answer.
This worked for my daughter who had been miserable for months and practically gave up eating. She went to the doctor and he suggested the vinegar. It worked immediately.
Nausea after eating, increased thirst, unexplained weight loss, and frequent headaches are potential warning signs of type 1 diabetes [more so than type 2 diabetes]. They also raise a red flag to a number of potential gastrointestinal issues. Bottom line is he needs to be evaluated by a doctor asap.
Ok. Thank you for your response. Will keep you posted.
I will tell him that if nothing else is conclusive. Thank you for your reply. Will keep you posted.