It would have been awesome if my mom did this when I came out. Obviously you want to understand what's going on, and you're undoubtedly worried about this, "Choice" your "Son" has made. Just remember to be supportive of what they do. They don't need your skepticism and thats the last thing their self esteem needs. They need to be listened to. Just remember to not doubt your kid cause they've definitely given this a ton of thought. Telling a parent something like this is usually something stressful for the kid themselves. It means that they are putting a lot of trust in you, and that you need to show the same trust with what steps forward they are taking to stop being miserable.
If you also seem to think that they, "Aren't feminine," just remember that that has nothing to do with what they feel like on the inside. Keep in mind that they are years behind socially, and that being feminine has nothing to do with how you feel biologically. Read the articles. Get all the facts, and love your kid.
-Xen