I still wouldn't bother telling about it. Although she seems like an ok person... she is a noble. Typically they will only care for their own benefit. If in another manga... he will surely regret his actions.
The way I see it, he's had to lie about who he was his entire life. He's a person from a different world that has his own experiences and trauma, but couldn't show who he was because of the problems that would bring him. He's downplayed his inventions and thoughts to avoid being used and abused by the world. He's essentially had to play a role in the theater of this world for 14 years. There's a lot of stress that would build up because of that. And even if he's an adult in mind, he's got a kids body. And finally there's someone he can be open about. Someone that cares for him and paid enough attention him where they realized that he was hiding something. Finally being able to be open about who he is must be scary, but relieving at the same time.
I still wouldn't bother telling about it. Although she seems like an ok person... she is a noble. Typically they will only care for their own benefit. If in another manga... he will surely regret his actions.