I am sorry to hear your dad is undergoing tests at the moment and it sounds like this is a worrying time for everyone involved.
As positive FIT tests are caused by blood in the stools that can be caused by lots of different things usually further tests are needed to determine exactly what is going on. This is usually in the form of a colonoscopy, or where this isn't possible a scan depending on the individuals circumstances.
As we aren't involved in your dad's care it is difficult for us to comment into why the doctors are sure he has cancer from the FIT test alone and this is only something the team involved can answer.
I hope you know more and that everything will be okay with your dad.
