A boy whose prospective adoptive parents are accused of killing him was described by one of them as "something like a vegetable", a court heard.
Christian died four days after collapsing at the Gays' home
The trial heard Ian Gay rang social workers with his concerns.
He and wife Angela, from Halesowen, West Midlands, are charged with three-year-old Christian Blewitt's manslaughter and with child cruelty.
He was in their care when he collapsed and later died. A high level of salt was found in his body, the court heard.
Christian collapsed at the couple's then home in Bromsgrove, Worcestershire, on 8 December 2002. He died four days later in hospital.
Mr Gay, 39, and Mrs Gay, 40, were convicted of manslaughter in 2005 but won a retrial on appeal.
Ian and Angela Gay have always denied poisoning Christian
Tests revealed a "significant" amount of salt in Christian's body and he died of a brain haemorrhage.
Nottingham Crown Court heard he had been taken from his natural parents under an emergency protection order at the age of one and put in the care of Mary and David Johnson, from the West Midlands.
They described him as a quiet and timid child.
Mary Johnson said: "He would sit playing with his cars. He loved playing with his cars. He was a quiet child. He was not boisterous."
He and his two siblings lived with them until they moved in with the Gays for a trial adoption period in November 2002.
The court heard the introduction period was described by social services as "textbook" until the trial adoption began.
Social worker Gillian Jones, who then worked for Sandwell Metropolitan Borough Council, received a telephone call, seven days into the trial, from Mr Gay who was concerned Christian was not doing the things they would expect of a boy his age.
He said he was not interacting with them like the other two children were and described him as "something like a vegetable".
Defence barrister Michael Topolski QC said after a chat with Ms Jones, Mr Gay realised he had not given Christian enough time to settle in and later felt guilty.
Social services had carried out an assessment and suspected Christian as having "developmental delay" or learning difficulties, he added.
The trial continues.