Berrin T., 48, and Christian L., 39, also stand accused at he district court in Freiburg of rape, aggravated sexual assault of children, forced prostitution and distribution of child pornography.
They are accused of having sexually assaulted the boy – who is now nine – as well as pimping him on the so-called darknet for more than two years.
Beyond a conviction and prison sentence, prosecutors are seeking preventive detention of the two due to the seriousness of the alleged crimes.
The stepfather has a previous conviction for paedophilia.
The case came to light following an anonymous tip last September, and led to the arrest of eight people who have been charged with belonging to an online paedophile ring.
Two of those arrested have since been convicted.
During the trial of the related cases, the stepfather admitted on the witness stand to the assaults on the boy.
He also made serious accusations against the boy's mother, who has so far been silent in court.
“I hope, even if this is unrealistic, that the boy’s mother will provide an explanation as to her motivation,” lawyer Katja Ravat, who is representing the abused child, said.
Ravat added that she wanted to be able to tell the boy “that the man [his step father] will not be let out of prison at any point in the foreseeable future.”