Should and how do i tell my friend i know his deep secret?

This is how it went -
the other night I held a Halloween party in my free house, with alcohol of all kinds and all the usual things you would find at a teen party. It was a ton of fun, some people went a bit too far with the booze but it was a still a night to remember: only now I have two reasons.

For some reason I thought it would be hilarious to jump in the bath because I was dressed like Charmander, I didn`t run any water, in fact I ended up falling asleep in there at what must have been 3am.

A lot of people had headed home by then and the only people remaining were the close friends I had selected to stay over, a handful of my great mates. My best friend was gonna share my room with me on the extra bed but because I had not turned up he decided to sleep on the sofa trying not to be rude. He is a really great guy, he`s soft, and sweet, really shy but a truly awesome and tender guy.

Anyway, I woke up and checked my phone and it was around 5am. I didn`t stir, I was just lying in the bath. This bathroom is the only one in my house, so if anyone wanted to use the toilet they would either try not to disturb me or go outside or anything. My best friend who I mentioned, came into the toilet, and saw me still in the bath with my hood up so he assumed I couldn`t see him. He was wearing a tiger onesie so to use the toilet he would have to unzip it all. I looked away but there are mirrors all around the bathroom and I saw something unusal.

When his onesie fell to the ground he was wearing something around his chest, that looked really tight. And he turned and sat down to use the toilet and pee. He then wiped, like a girl.

After flushing the toilet he stood up and zipped up his pj`s, then left. I stayed in the bath for another hour just thinking about it. He is always going to the Doctor, at least once a month and he asks that one ones inquires about it because it is an embarrassing issue. And seeing him like that I understand it all now.

I`ve only known him for three years but I really love him as a best friend, and honestly, I`ve always had the tiniest crush on him. He is a such a sweet guy, and only know I`ve found out that he is actually transgender, if that`s the correct word?

He has to be getting some kind of treatment because he does sound and grow hair like a boy, so somewhere along the line he might be having an operation if I`m not mistaken. I`m not sure if he hasn`t come out to me because I`m a guy, and he thought it might be uncomfortable, but he has done really well being strong and making friends as the great guy he is. I would never have guessed.

I want to tell him that I know about his secret and that, I want to be supportive to him and help him any way that I can, but how do I break it to him? I`m sure it`s a sensitive issue and I don`t want to upset him, I jsut want him to know I`m there for him 100%.

Any advice? Thank you.