It does not make sense to look into a german database (guess FN Jahrbuch) it is too vague as it does not contain the complete population and especially if he is already in the USA for a longer time one may not find him.
If you really want to know take pictures of him from all sides so that the markings can clearly be seen. Than together with the numbers under the brand send it to the German Hanoverian Verband and they may come up with a variety of horses.
If so one can pull hair from him, and have it DNA tested whether he could be by one of the stallions that are the fathers of those the Verband has found.
If you contact them (Homepage was already posted above), they may help you.
You could even try to get a set of papers, but there are a lot of costs involved and for a gelding I would not go that hassle.