Balram Patel died despite his father repeating that the medication he was receiving was ineffective.
A man would still be alive if his parents had been able to get a second opinion from another doctor, they say.
Balram had multiple disabilities and was developmentally delayed. He was born with half a functioning heart and had liver cancer at the time of death. It read: "This is a failed discharge. They have not put systems in place for this to be supported in the community. We have to now know the orals are not working. You taking him back is a failed discharge. The discharge was not robust enough."
He said: "The NHS has been there for us for all of my son's life; Balram was here for 30 years because of the NHS. But because of the same NHS he passed away, before his time and in pain.