He allegedly confessed to the point-blank shootings, on Sunday, after raising the alarm.
The boy, described as being in a trance when found by police, originally said he found the bodies when he returned home after an evening out.
His three-year-old half-sister was found asleep and unharmed, near her mother's body. The boy was reported to have hidden her under a blanket so that she did not witness the killings.
The other victims were the stepmother's parents and another couple, friends of the family.
Police named the father as Yevgeni Polevoi, a wood merchant, who settled at Louveciennes in 1993.
Many guns, some of them collector's items, were found in the house. Several of the weapons were used in the killings, police said.
The victims, aged between 50 and 70, were apparently shot as they prepared to go to bed.
Police said the boy complained of ill-treatment by his father, who remarried when he arrived in France.
Investigators first thought the killings were the work of the Russian mafia or a gangland settlement of accounts.Reuse content