It's not unusual for parents to be tolerant of things like religion and sexuality with others, but not ok with it being their children. Think about it this way: transgender people are a group he doesn't understand, but knows are persecuted. Why would he want his child to be one?
If you're still in high school, you should talk to the school psychologist. At 17, you're going to have the problem of not having alot of autonomy, but you have the luxury of time. Your bones don't stop growing until 25, and as long as you do it before getting set on a career path or settling down, you won't have to rework an entire life.