I'm going to make a meta-comment here.

I think that your ultimate goal should NOT be to convince your dad that you are right and he is wrong. If he eventually changes his mind, he's going to have to do that on his own. Debates just don't change participants' minds very often.

Instead, your goal should be to make him respect your beliefs as genuine.

Christians generally respect people who are genuinely seeking truth, in part because the Bible promises that "those who seek will find". The good news is that you ARE legitimately seeking truth, so you should be able to convince him of this.

Hopefully you already have a good relationship with your father based on mutual love and respect. You want to build on that and preserve it as much as possible. He is going to be your dad for the rest of your life, and how you interact with him now is going to determine in part how that relationship develops.

More practically: It sounds like you aren't sure exactly why you've changed your mind, and are having difficulty articulating it. Nobody on this site is going to be able to articulate it for you. Rationality is a method, not a conclusion. So here is my suggestion: do a stack-trace on your change of belief. It happened, so it is causally entangled with some set of arguments and evidence you encountered. Go back and try to figure out what caused you to change your mind. Reconstruct as best you can, in your own words, as exactly and precisely as possible, why you changed your mind.

This exercise will help you to understand what you believe and why. Discussing this with your father will be grounds for a future relationship based on mutual love and respect. That should be the goal here.

Last piece of advice: spend some time with your dad doing something other than arguing. Go to a baseball game or something. Try to get some father-son time where you're not just talking about your beliefs. You want him to get used to the fact that you're the same person, and you don't want this to dominate your relationship.