The boy, known as Goran, required immediate medical treatment and has been suffering post-traumatic stress disorder.
Brazen predator Amir A even went back to the swimming pool after the rape and was playing on the diving board when police came to arrest him.
Confessing to the rape in an interview with officers, he said he knew what he did was wrong but did it anyway because it was a "sexual emergency" as he had gone four months without sex, according to the Local.
In the initial court hearing, he was found guilty of sexual assault and the rape of a minor and was sentenced to six years in jail.
But the Supreme Court rescinded the sentence and ordered a retrial. The court's president Thomas Philipp called the verdict "watertight" when it came to the sexual assault of a child, but said rape could not be sufficiently proved, said the Local.
The higher court said it should have been ascertained whether or not the victim agreed to the sexual act or whether Amir A had acted against the will of the boy.
A second trial for the rape is expected to take place next year, but the attacker is likely to remain in custody until then.