While I tend to think I would move toward connecting with a half-sibling, another part of me asks if there would be gain or loss to the other party.
If you cannot give a good report on his father, the person he would want to know most about, then it might not be a great idea.
Imagine a summary like this: "Your father was a huge hypocrite that belonged to the Jehovah's Witnesses."
I hope you have more good things to say about him, without a bunch of spin. If not, in my case, I might let this go.