Ryan Morse, from Brynithel, Blaenau Gwent, died on 8 December 2012 from undiagnosed Addison's disease.
Dr Leonard Peter told Cardiff Crown Court Dr Joanne Rudling, of Cardiff, did not ask enough questions about Ryan's condition hours before he died.
She and Dr Lindsey Thomas, 42, of Tredegar, deny manslaughter.
Dr Peter, who has worked as a GP for 41 years, told the court 46-year-old Dr Rudling, one of his GPs, should have shown more curiosity about Ryan's state.
Zoe Johnson, defending Dr Rudling, put it to Dr Peter that medical notes suggested Ryan was "feeling a bit better from a temperature point of view" the day before he died, but still suffering from sickness and diarrhoea.
She said Dr Rudling had offered him the chance to see a male doctor on Monday after she was told of a change in the colour of Ryan's genitalia, something she felt was a symptom of puberty.
"Looking at the record I'd be concerned that a child had serious symptoms in the morning, sickness, delirium...and had now developed a symptom I'd never seen before (the skin colour change).
"Doctors are curious, or at least they should be curious...and you can't risk manage it if you haven't seen it." Dr Peter replied.
"She had a lot of this information in the records and nor did she seek the information that wasn't in the records.
"She didn't take an overall view of what was happening...this was a 12-year-old whose mother was changing him (because of such severe diarrhoea).
"Had she seen it, she wouldn't have thought it was puberty, she'd have thought 'I've never seen anything like this in my life.'"
The court has previously heard Ryan's death could have been prevented if he had been examined at any point up until a couple of hours before it happened.
Both doctors deny manslaughter through gross negligence.
Dr Rudling also denies attempting to pervert the course of justice with an entry made in Ryan's medical notes two days after he died.
The trial continues.
A GP should have been more curious about a 12-year-old boy's symptoms and seen him "as soon as possible" the day before he died, a court has heard.