I really want to help my dad?

My dad is extremely overweight. He doesn't really eat unhealthy food, he just eats a lot. I've tried to talk to him about it but he just get's angry. One time he actually agreed to go on this diet that really worked for me. But on the first day I caught him eating a doughnut and he decided that he didn't want to do it anymore. the thing is he is not even willing to TRY anything. It seems like he is always eating, for eg he'll have two mive bowls of whatever we are eating for dinner then straight after dinner he goes and eats. I know that he's not hungry, he just eats for the sake of it. I really want to help him but I don't know how. Please help! thank you!