Hi Vicky, since 1876 all births, death an marriages in Germany had to be registered at the civil registry office (=Standesamt). You might ask the current Standesamt firstname.lastname@example.org, whether the could sent you a copy of the birth entry. If they still have it normally they will sent it to you for a small fee. Otherwise they might tell you which archive you have to ask. Before 1875 you could have a look at the parish books. If he was protestant you find the parish books of Oberesslingen at www.archion.de/en . To have a look at the scans of the church books you have to subscribe to the website (e.g. 1 months about 20 Euro). It is really worth it (if you can try to read German especially in old writing - one gets used to it). But first I would start with the civil registry office.