Contact the relatives of this person, if you know them or you know where they live, and ask them where he was born. When dealing with such question words be sure to introduce yourself and tell me what purpose you require such data. If you are a representative of a state institution, you will probably answer this question. If you are an individual, the reason must be sufficiently weighty in order to give you a comprehensive answer.
Continue your research by contacting the archive of the registry office, where, according to your assumptions, you can store data on place of birth of this person. Please note: in the books of account of the institution entered any information related to birth, conclusion/termination of marriage or death, but in the evidence it may not be. So if you sent a request to the Registrar, and you replied that he did not have such information, you may request an official written refusal with which you will be able to go to court. It is likely that employees in this case will come towards you.
Make requests to the district, regional and national archives, depending on what information about this person you have. So, if you know that he was being called up for military service, make a request to the military Commissariat or the archives of the Ministry of defence, and if he was in custody in the archives of the Ministry of internal Affairs, GUIN, FSB (provided that the funds where it can store this information, open).
Visit website where you have a constantly updated database of Russian citizens. Select the name of a wanted person according to the alphabet and learn about his probable fellow countrymen. You will also be able to ask the specialists working with this resource, you are interested in the question on the forum.