niego is third form , we use n.... forms of pronouns after prepositions. ( declension of jego in Polish- English version does not have niego.
Also http://www.skwierzyna.net/polishgrammar.pdf has on page 22of the file CASE FORMS OF PERSONAL PRONOUNS.
Personal pronouns show a full range of case forms, summarized in the following chart. Longer forms jego, jemu ciebie, tobie, mnie are emphatic, and occur automatically after prepositions. Also after a preposition, forms beginning in i or j lose j and take a preceding ni-: bez + jej = bez niej, dla + ich = dla nich.