Simon Ankers, 21, said he sniffed the knock-out drops so he could forget his troubles after his parents banished him from their home. But he allowed Bobby Joyce to sniff or drink the anaesthetic and the boy died, York Crown Court was told.
Mr Ankers, of Selby, North Yorkshire, denies two separate offences of the manslaughter of Bobby - one by an unlawful and dangerous act and the other by gross negligence.
He admits stealing Halothane from the Selby veterinary surgery where he helped with operations, but denies administering a noxious substance with intent to endanger life in October 1992.
Mr Ankers, the son of a businessman, told police he had been banished to the garage loft because his parents thought him an academic failure and could not accept his love for motorcycles.
He took the drug to help him sleep but had it with him when he called at the Greenacres Caravan Park in Selby, where Bobby lived with his parents.
Mr Ankers awoke and found Bobby slumped on the floor, but despite frantic efforts by himself and others, including the boy's father, they were unable to save him.
The trial continues today.