Thank you for your question. The answers are:
1. According to German law you are obliged to pay a form of child support even after 18 years of age, as long as the child pursues university studies;
2. On the other hand, as the person with parental responsibility, you also have the right to be informed of your child, his whereabouts, address, university being pursued etc. This includes demanding his grades (marks / tests) as well, and knowing his address and even contacting him;
3. The attorneys do not have a right to search for you in the US. If you refuse to pay and don't give your address, they can sue you in German courts, and the court will decide in your absence. Once the decision has been taken, it can be enforced in the US, through a US bailiff (who will probably use a private investigator to search for you). This is just a civil matter (i.e., the most the bailiff can do is sell assets you own or garnish your accounts), and it would take some time. So, the most that you expose yourself to is having to pay according to the court order;
4. My suggestion is, however, to continue asking the lawyer for documents and contact details of your son, as well as for any court documents, if he does start a trial. If there is a trial, the best thing to do would be to hire a German English-speaking lawyer (these can be found here: https://de.usembassy.gov/u-s-citizen-services/local-resources-of-u-s-citizens/englishspeakingservices/). A local attorney would have access to the case file, and would be able to make copies of documents, including the contact details of your son, enabling you to make direct contact.
I hope my answers were useful and look forward to your rating, which is essential to my activity.
Dr I L Vlad