The brother of Hamzah Khan told a jury he saw the four-year-old eating the contents of his own nappy.
Qaiser Khan, who was giving evidence today at Bradford Crown Court on the third day of the trial of his mother Amanda Hutton, also said Hamzah's buggy "stank" of urine.
ITV News correspondent Damon Green reports:
The 22-year-old said that the young boy "appeared neglected" during his visits to the house between December 2008 and March 2009.
Stephen Meadowcroft QC, defending, asked Mr Khan whether he was exaggerating what he had seen at the house because of his bad relationship with his mother.
Mr Khan denied that was case and said when he was 13-years-old he was made to drink "mouldy, off milk" by his mother as punishment.
Ms Hutton would spend a lot of her time drinking, disappearing upstairs "for most of the day", Mr Khan told the jury.
Hamzah's body was found in a cot in his mother's bedroom by police in September 2011.
The four-year-old died 21 months before he was found, Bradford Crown Court was told.
Ms Hutton denies the manslaughter of her son.
Earlier, the court heard how Hamzah's father demanded police check on his son a year before his death from alleged malnutrition.
Aftab Khan, 45, was being interviewed by Bradford police in December 2008 over an alleged assault on Hamzah's mother.
The interviewing officer told him to contact social services, to which Aftab Khan replied: "I'm going to."
He said of Ms Hutton's treatment of the child, who died a year later:
The court heard that despite Aftab Khan's promise to contact social services over the health of his son, there is no record that he did so.