There are just 54 entries for Langejuergen in dastelefonbuch. It could be worth a few polite letters just to ask if anyone has any knowledge of this. Only costs a few stamps and the worst that can happen is no reply - and it has worked for some. The second possibility is that you seem to be sure he was in an UNRRA camp. UNRRA records are held by the World Health Organisation why not write to them and see if there is any record of how the child ended up with them, who the father really was and where he came from. That could pinpoint a district which could help.