Hm, it's not really a good thing when one parents has your influence over another :/ What you say about your dad might be how you feel, but you probably shouldn't say it with such cold blood. Just tell him how you really feel without all the harted mixed in, becuase let me tell you, it's only going to make matters worse. And it's so easy to do/say things you'll regret later in life, weather you think that will happen or not. My parents are divorced and I have a step dad who loves me, but I love my meternal father, too.
I'm not sure what kind of realtionship you have with your dad, and in truth, I really don't want to know. But just remember you do not know what's going inside your father's mind. What you say could seriously hurt him, and that's not a "good" thing. No matter what kind of situation this is, all i'm asking is that you respect your father while doing this In truth, it's best just to pray for him so he'll get better.