Given the symptoms that your father is suffering from and not getting relieved by medications prescribed, most likely he may be having Reflux of food or Dyspepsia.
Next Steps
Better to have a trial of some Antireflux medications after consulting with your doctor. If no improvement, he may need to undergo upper gastrointestinal endoscopy or any relevant tests advised by your doctor.
Health Tips
Consult with your doctor before taking any new medications.