The 50-year-old Bundeswehr soldier, who has been suspended from his post, admitted during a private hearing that he sexually abused the boy twice, together with the child’s stepfather, said presiding judge Stefan Buergelin outside Freiburg district court in south-west Germany.
The suspect also said he filmed the acts and shared the footage with others.
The boy, now aged 9, was allegedly raped by men from home and abroad over a period spanning more than two years.
The soldier, the boy’s 48-year-old mother, and her 39-year-old partner are among eight suspects in the case.
All are in custody and will face trial separately.
In April, a 41-year-old man was sentenced to 10 years in jail for raping the boy, plus continued detention following his sentence in the interest of public safety.
A verdict in the soldier’s trial is expected in May.