I hate when people say that. You can quit a hobby. I would not say most ( some here may agree but it isn't a great number). If it were you could quit or redirect that energy to something else. Saying this may be a hobby to a young person, belittles a lot of things. This thread asks how dad could have helped, a dad calling it a hobby is like sticking your head in the sand.
MJy dad is having issues with this. He doesn't understand why his "perfect and educated son" would want to change into a (ugh0) daughter. He is old school where being a man is the pinnacle of society, being a man who does manly things even more so. He "never wanted a daughter" when I was younger, he had two sons. Me, a TS, a gay and one who now drinks too much. Any of this from the home life? Maybe. We never said "I love you", nothing was ever good enough (You could have done that better or yeah you won the game but you messed around so long). So maybe there was more to just the nature complex.
I say being TG is nature but reinforced by nurture. In fact everything in a child's life can be put in that category. If you water a seedling it grows. If you ignore it or spend little time with it, it doesn't. In this case, learning all you can and supporting who he is at this time in his life is the best you can do. Things may change but in this case, while he is exploring this he knows there is a llife line.
Good for you Dad, and good luck. Lerner and Lowe wrote a song that was about how to handle a woman but it is pertinent to how to handle any relationship. I will paraphrase
How to handle a (child) there's a way said a wise old man, a way known by every (child) since the whole who human race began. How to handle a (child) mark me well and I'll tell you sir....the way to handle your (children) is to love them, simply love them love them love them.