David Bell Bryson, 71, earlier told a court that he was sorry for killing the boy in a state of uncontrolled anger, with claims the child had tortured his dog.
But earlier this week he told magistrates in the Danube town of Vidin that he would plead innocent to the murder charge.
He told the court: "I will not plead guilty. I have not seen the kid on that date and I have not killed him."
Stanislav Mirchev, who used to carry out chores for the accused, was allegedly strangled to death in the village of Lagoshevtsi, close to Vidin, in January this year. He was discovered in undergrowth on the outskirts of the village following an 18-day search.
Prosecutors have claimed that Bryson failed a lie detector test and later confessed to the crimes.
Bryson will now face trial in Bulgaria following his change in plea and could face up to 20 years in jail.
It was reported that a guilty plea would allow him to serve a lesser sentence, possibly in the UK.
Bryson was born in Larkhall, South Lanarkshire, but studied and worked in Germany as an aeronautical engineer before retiring to Bulgaria four years ago. He still has family in Scotland.
Prosecutors are expected to demand that Bryson is sentenced to life without parole, saying the murder was committed in a particularly cruel manner.
An earlier court hearing was adjourned as Bryson and the boy's parents launched court negotiations over compensation.
Bryson will remain in custody as the case progresses as a judge ruled that he posed a flight risk.