The man’s parents were found dead at their house in Hedemora, Dalarna, on Saturday afternoon.
“He says he did it, so our job now is to provide evidence that it happened in the way that he says,” said Sven-Åke Petters, spokesman for Dalarna police.
Police and prosecutors are so far giving few other details of the killings.
“We do not currently wish to release details of his motive and method, out of respect for the family,” said Petters.
He said that the son had made a partial confession, but said that his story “has aspects that suit him better,” and that might indicate manslaughter rather than murder.
It is now believed that the murder took place on Thursday. The dead couple, a woman aged 62 and a man aged 59, were found in their house by police after another of the couple’s sons said he had not been able to contact them for several days.
Police forensic teams were expected to complete their examination of the scene on Sunday. Post-mortem examinations will be carried out on the couple’s bodies on Monday.
Police are so far refusing to say how the parents were killed, although news agency TT reports that their bodies “bore such clear signs of physical violence that it was clear a crime had been committed.”
The 18-year old was previously unknown to the police and has no previous convictions.
Sven-Åke Petters says that no clear motive for the crime, such as a fight, has been ascertained from their interrogation of the teenager.
“Everything seems a bit diffuse,” he said.