Hazard, it's awesome that your father is taking it so well.
I've run across a number of trans people whose sexual orientation shifted somewhat, but very few who completely flip-flopped and went from completely gay to completely straight, or the other way around. You know what I think? I think most of them felt a lot freer after they started transition. Maybe they were just starting to loosen up and admit to things they were afraid of, or try things that they were afraid to try. So many of us keep ourselves locked down, and transition frees us up. You could open a new thread and find people who went from gay to straight or straight to gay, and read their theories on why it happened. I wonder how many responses you would get. Hm, maybe I'll start a thread like that.
Almost everyone I've seen has stayed essentially the same. Some folks who are bi or pansexual have noted that they like men more or like women more now...but they were bi to begin with. Some people who were attracted to one sex only have found that they would at least consider the other sex. But that's not a complete 180-degree shift.
I suppose it's possible that taking T has some effect on orientation, but I've never seen any evidence that these shifts are from hormones.
At some point, those of us who DO start hormones have to take some kind of leap and decide that the potential benefits outweigh the potential harm. But I don't think you have much to worry about with regard to orientation.