I'm going to tell my father he eats too much
What is your reason for thinking that he eats to much?
I've been trying to count his calorie intake for a couple of days.
That is not a good way to go about it. It's rude to tell someone that they eat too much. You should express concern, but not tell him what to eat or how much to eat.
Dude, I'm trying to help him lose weight.
Well try to tell him this in a more respectful way so you do not come across condescending to him. I would assume he knows when he is eating too much, so you may be better off simply offering help to him instead of pointing out the negative.