Tanent right in case of evacuation

Hi, My father age about 68 yrs living in a rented 2 room flat for near about 15 yrs and he is also a patient of kidney,blood sugar,parkinson,and heart diseases.My sister,a school teacher (38yrs, married with no issue and not living with her husband though divorce has yet not done) living with him. Now days owner of the room asking him to evacuate the flat giving him a personal reasons i;e owner said that " he now wants to live in this flat though we know that they wants to give the said flat to other with higher rent value. Though owner given a verbal notice to evacuate but my father is now living with heigh tension. Pls suggest me what to do and what law said. Address of my father:- Arambagh,Hooghly,west Bengal.India. On 20/08/16 son of landlord threaten my father by saying that " they will occupy one of room of said flat showing illness and for better treatment of his father and after that they will forcibly evacuate the said rented flat where my father lives." PLEASE SUGGEST US THE NEXT STEP TO DO BY MY FATHER.The landlord has property and very close relatives lives within the premises of the said rented flat and landlord lives within 18 km of the said rented flat. Father had legal contract for rent where time period of rent is not mentioned and my father gives rent regularly.