symptoms of cancer

my dad's stomach hurts him alot and he has abdominal pain also. furthermore he has a tumor in his mouth that has been recently removed ( sucessfully) but regarding his stomach nothing from blood tests and other tests show any thing . the only other tesst left are the ct scan and kidney test, should I be worried that he is cancer? I need to know cause I have a 9 year old sister and we are both really scared on losing him