Father working at senior position but denied promotion by bank

My father has been working in a cooperative bank for 36 years. In 2011 when the General Manager of the bank retired, he took the additional charge of GM and worked as GM(additional) for 45 months, as he was the senior most person in that bank. His qualification is M.A through open university. He has not been promoted as a regular General manager.Last week another A.G.M was offered the G.M promotion. Please help him how to handle this situation. He has not been paid extra for the additional charges he took for 45 months and not been promoted as well .Please suggest the steps to be taken. Thanks