Retired Government Employee Denied Leave Period Regularization

My father was a government employee under Telangana Housing corporation Limited and retired in September 2019 as Deputy Executive Engineer. During his last 10 months' tenure before retirement, he was bedridden and advised to take bed rest due to back pain and he has a history of back spine surgery. After his retirement, he tried to get regularization of his leave period with a Medical and Fitness certificate from Civil Assistant Surgeon. He is entitled to 187 Medical Leaves from his service for Full Payment of salary and remaining will be entitled to half-pay medical leaves. He applied for the same but the manager of his organization is denying for full payment of leaves saying he dint apply for a medical leave formally before and he is considered as absconded. My father has tried to convince him several times that after producing a Medical certificate, a prior Leave application is not required but he threatens for an inquiry. What legal course my father has to take to claim his rights.