By who’s law did he have to affirm his American citizenship. By German law, then as an American citizen he did not have to do anything. If Germany asks you to do anything, the appropriate answer from an American citizen is KMA. James Madison would back your son on this course, as he did those dragged off of US ships by the Royal Navy who did not renounce their UK status when they became US citizens.
Truthfully, I don’t remember. It all took place around the time he had to register for Selective Service.