I know exactly what kind of situation you are in.
My dad was an alien too. I mean he's quiet, I didn't know anything about him. He's also from another country so he is isolated in Poland. He never talked about his feelings. I didn't know what he does, who he is, even what job does he have. I'd seen him every sunday for a few short hours. So I think I know how hard it is. Cause it is. It's just you don't know this man and even though it's your dad you just feel like it's a stranger which you are bounded still.
You should always do it with an honest conversation. I always encourage people to do that.
But well, I did this the other way. My mom did. She just told him. Just like that. So the problem solved thogh I told her I'm gonna do it myself. But she doesn't listen. I think half of my family know because she couldn't keep her mouth shut. But it still did the job. He texted me that he knows about my "otherness" but that doesn't change that he loves me. So yeah. If you have such a big problem then that's one way.