I'm writing a story and i need some to get some medical things correct.

so to go in depth, i'm writing a story where one of the main characters is 17 and gets diagnosed with stage iv stomach cancer. i know that people usually don't get stomach cancer until they're 60ish but i needed something that he wouldn't survive from.
if there's a different kind of cancer that 17 years olds can get, let me know.
he's anorexic, and would throw up any food he ate, and so before they got him to the hospital, he had horrible abdominal pain.

i don't know if any of this interferes with that but i thought you should know if you were to answer this.

so if you know of a cancer that can be detected at the very last minute and have no previous symptoms, is in the abdominal region, and can be deadly to an underweight 17 year old, that would be great. but if there's not then i'll just have to say that it's not