I'm not talking about "fixing" your Dad or making him see reason. Time is short.
I'm talking about your honest anger and resentment of being treated unfairly by your Dad from the moment you decided to choose your Son's Dad against your own Dad's wishes. It isn't fair what your Dad did and it isn't right. BUT is that the real problem?
The real problem as I see it is that your Dad won't open up and hear you. Why? I think you are assuming it's all about the prejudice, which lets you put the whole blame on your Dad, but your Step-Mom said something different.
Can ANY part of it be your honest anger and hurt causing you to lash out even as you are reaching out to your Dad? IF ANY of it IS that, then you have more personal control in this situation than you realize.
You have every right to be mad, but if you want to get the conversation going again before Dad dies, go to a pro. A doctor doesn't operate on him/herself and you don't drop your trannie in the back yard unless you know what you're doing, have at least a swing set cherry picker and the right tools to do the job.
IF you can't get Dad to listen to you on your terms, change your terms, for your Son's sake. I think the past loving foundation you two had can bridge a lot of this, but you need a few tools to remove the barriers that have been put in place over the years. Go get the tools. All you have to lose is a little emotional baggage that's not even useful anymore.
I'm pushing you. I know. I'm sorry. But getting something like this right or getting it wrong with a dying parent can completely change your life and my hope is that you get it right and at least have proper closure knowing you made the kind of effort that gave your Dad no alibis to put it on you.
Peace.