Can you tell me what forms I will need for the cruise line to prove that I am allowed by the parents to do so?
Besides his passport you will need an original notarized letter signed by both parents which states that you are authorized to be the child's guardian while on the cruise. I have no idea if Germany has notary publics, but something equivalent to it would be needed. The letter needs to have an official seal of some sort. I'd also have it written in English.