I lost my own father to aggressive lung cancer in 6 weeks . He also felt sure he would beat it although he had to know as it was in operable it was unlikely . I now have breast cancer myself and my advise to you would be to encourage his positivity. Diagnosis is a lonely time he does not need his mind full of fear he also may need to think he will survive as he cant face knowing he may not . Give him that and spend your time with him wisely x good luck with everything