What to do if my dad is vomiting every time he drinks water or eats anything?

See a doctor: If your father is vomiting everything back up, he will dehydrate quickly. Please take him to the hospital emergency department for evaluation and treatment

Needs medical eval.: Vomiting after drinking or eating can possibly be a sign of obstruction of the esophagus or upper gastrointestinal tract. Abdominal bloating would go along with this. His history of radiation therapy could be a factor if it was to the upper chest. Symptoms like this require thorough evaluation by his medical doctor as soon as possible. Hope this helps.