I don't have a Kindle, but I bought one for my mother. When it was sent it was already registered to me. They sent me an email telling me how to deregister it if it was a gift. I did, and when my mother opened it, it was ready for her to register it to her Amazon account.
I don't think there would be a problem if your stepdad didn't like it. He'd just deregister it, then you could put it back on your account.
That said, since he has arthritis, you'll need to open the box for him. I had some trouble opening it for my mother. It has a nasty pull tab down the side of the box. I had a hard time getting a grip on it to get it started.
At first my mother was intimidated by it. She charged it up, but didn't turn it on for a week. I finally harassed her into it, she got it registered, bought a couple of books, and now she's fine. I hope.