Jack Dunn, 17, was sent home from A&E at the Royal Glamorgan Hospital in South Wales with laxatives after a constipation scan.
The teen was told to get a good night’s sleep but the next day his father Keiron found him dead in bed at his home in Rhondda.
A post-mortem found that Jack died from a condition called ketoacidosis – an illness that causes build up of acidity in the blood.
Mr Dunn said his son could “barely walk” because of how much pain he was in.
He told The Mirror: “When Jack was finding it difficult to breathe the doctor thought it was probably anxiety because Jack was anxious about being in hospital.
“Finding my son dead in his bed was the worst moment of my life. I believe Jack would be alive today if a few more simple tests had been carried out.”
Mr Dunn believes his son would still be alive if doctors had given him a glucose and salt saline drip.
He added: “Jack’s white blood cell count was up so that was an indicator something was going on in his body that wasn’t right.
“If they had tested him for ketones he would probably be alive today.”