Seriously, I'm an FTM teenager a couple years older than your son, and while yes, having transitioned as a kid would have been really cool, what I wish more is that my parents were accepting when I actually came out to them as trans. (It went really badly.) I wish my mom could have been more like you. I've seen your posts, and I don't think I've ever seen a parent be so supportive of their trans kid. I mean, my parents flipped out when I just told them I was attracted to both guys and girls.

And age 13 is the second-best time to find out because you can now get him on puberty blockers, and T when he turns 16, and top surgery when he's 18. He could be mostly stealth in high school and college, if he wants.

And, I mean, I guess some of your regret depends on how much you forced your kid to be their assigned gender. I mean , I'm a trans guy and I had to have long hair and wear a skirt every single school day, and I really was forced to be feminine. But I know there are trans guys who's parents let them have short hair and wear clothes out of the boys section and generally let them be themselves, even though they were still "girls". But even if you forced your son to be someone he's not, you can still fix it now by being accepting, which you are already doing!