The boy was apparently with two companions aged 12 and 13 when he set alight a mattress being used by the man in the northern town of Novi Sad.
The man died of his burns in hospital on Wednesday morning.
Police said the boys were questioned while their parents were present and then freed because they were under 14, the age of criminal responsibility.
Serbian news agency Tanjug quoted juvenile crime police chief Olivera Zecevic as saying proceedings would not be launched "even though police found out who did it".
Ms Zecevic said it was important that parents knew where their children were and took responsibility for guiding their behaviour.
Police should investigate whether there was a basis for prosecuting the parents in this case, lawyer Milan Alanovic was quoted as saying on Serbia's RTS TV.
The age of criminal responsibility varies widely across Europe - from 10 in England, Wales and Northern Ireland to 16 in Russia.
Eight: In Scotland children can have a criminal record but cannot be prosecuted until 12. The Scottish government has promised to raise the criminal age to 12. In Estonia, children aged seven or older can be sent to a young offenders' institution
Ten: England, Wales and Northern Ireland
Twelve: Netherlands, Turkey and Hungary (for violent offences)
Thirteen: France has no criminal age of responsibility, but before 13 only education measures can be used
Fourteen: Serbia, Cyprus, Italy, Malta, Spain and Germany
Fifteen: Czech Republic, Denmark and Iceland; In Austria no criminal penalty at 14 or 15 unless misconduct is "serious"
Sixteen: Russia, with some offences from 14
Serbian TV said the boy set the man's mattress alight at approximately 03:30 on Wednesday (2:30 GMT). The fire quickly engulfed the man's clothes and he died despite being taken to hospital.
The victim was described as 52 years old.
Police are continuing their inquiries into what happened and what further steps can be taken.
A 12-year-old boy who killed a homeless man in Serbia will not be prosecuted because he is too young, reports say.