The season 5, 12th episode "Stefans Geheimnis": When Stefan (Christian Blümel, 14) realizes that he's gay he tries to cut his penis with a razor blade, standing nude in the bathroom (side/rear).
Near the beginning the autistic boy (supposed to be 11 y.o., but obviously older) is sitting nude on the floor, seen from the side. Later he is drawing a lot of pictures of penises on a wall and on a sheet of paper.