Prosecutor Emma Olsson, who heads the preliminary investigation, said the woman who was held on suspicion of unlawful deprivation of liberty and grievous bodily harm.
Olsson said the 41-year-old son was found by a relative who then alerted authorities.
The woman has denied wrongdoing, Olsson said, adding that the son was admitted to a hospital to be treated for "the physical injuries he had when he was found."
"We cannot go into what type of injuries" he has, Olsson said.
Swedish newspaper Expressen said the man was found Sunday by a relative who had learned that the elderly woman had been admitted to a hospital. The relative went to the flat, found the main door unlocked and entered it.