Hi Ms Swathi,
Thank you for posting your query. 
I have noted your father's medical details. I am glad to note that he has withstood two brain surgeries well. 
The recent 
fever may or may not be related to the surgery. Please get him examined by the neurosurgeon to look for any infection at the operation site. If that is ruled out, then, we should look for other sources of infection such as 
pneumonia or 
urinary tract infection.
I am sure he would improve with 
physiotherapy and appropriate medical treatment.
I hope my answer helps. Please get back if you have any follow up queries or if you require any additional information.
Wishing you good health,
Dr Sudhir Kumar MD (Internal Medicine), DM (Neurology)
Senior Consultant Neurologist
Apollo Hospitals, Hyderabad, India
Click on this link to ask me a DIRECT QUERY: http://bit.ly/Dr-Sudhir-kumar
My BLOG: http://bestneurodoctor.blogspot.in