Prosecutors said the defendant habitually surrounded himself with young boys between 1998 and 2013 in order "to arouse himself".
The victims were typically aged between 9 and 13 years old, the court head on Monday.
Many repeatedly came to visit his home in the Lake Tegernsee area, 50 kilometres south of Munich, because the defendant "allowed them behaviour that was prohibited elsewhere", prosecutors said.
The boys were frequently persuaded to remain naked during their stay and engage in kissing and cuddling. One boy aged around ten was however said to have been subjected to forced sexual intercourse, media reports said.
More than 40 of the 406 counts were deemed to be serious violations.
The defendant is said to have used cigarettes, alcohol, online gambling and computer games to entice boys to his apartment, where he built up a 'father figure' relationship with them.
He fostered additional trust by taking them on excursions and cinema visits.
During the hearing, which is scheduled to take place over seven days, the man identified as Ulf G. denied all the charges, claiming that he had only "cared for the boys".
The defendant reportedly rejected a plea bargain that would limit his jail sentence to a maximum of eleven years if he pleaded guilty and agreed to undergo specialised therapy. His lawyer rejected the terms on his behalf as being too harsh.
The man, whose parents came from the former GDR, spent part of his childhood in Tanzania where his mother worked in the East German Embassy.
He was described as being disruptive at school and had a record of disciplinary action during his military service. He was last employed as a chef in a café in Bavaria.