Mentally not fit. Left. please suggest!

As you have rightly pointed out, your father being 70, seems to be under constant fear of death. Having witnessed your mother's death he is expecting death to come in any form, hence under constant pressure to fight it off. Elderly people could be helped to accept and integrate the idea of death into their life. Please talk to him whenever possible about his past and how his life has been meaningful so far. Stress on his achievements, could be at work or at raising so beautiful children who truly care. Also help me integrate memories of your mother and how she supported him in every way. Once he feels certain contentment about his life, he wouldn't be so fearful of death. Do consult a psychologist/psychotherapist/psychiatrist for one or two sessions of counseling which should help him further to gain acceptance. Good Luck.