Investigation period of police

Sir, my dad is falsely framed into a murder case. Its been more than 3 months now and police has found nothing substantial and concrete against him except for the rumours. He is a government employee and is suspended due to his custodial interrogation of 8 days remand. He was out on bail and was out of station after filfilling the conditional part of 1 month of the bail period. He is 56 yrs old patient of various ailments and is bed ridden till date. The police had applied for his bail cancellation and has cancelled his bail illegally. Now he has asked for time to reach station but they are not cooperating. They have also prayed for extension of investigation period beyond the prescibed period of 90 days. What is the provision of law in extension of investigation period? For how much time and for how many times can this period be extended? Also,since the case was a conspiracy, the police has not done any autopsy of the victim so no post mortem report is there with them. Can we use it in our favour? Also how can we stop them from extending the period of investigation? Please help.