SOLVED. YA - A novel about a voiceless boy, math genius, and a girl (teenagers), read in 2000's [s]

[Patrizia]

In the book the boy, who doesn't talk but is very smart, helps the girl with math homework. They fall in love, and in the end she discovers why he doesn't talk.
(...sorry for my english...)

For more details...
(view spoiler)[ Actually the boy is able to talk, is a psychological trauma caused by the jealous older brother. When they were children his brother try to kill him (drown in a pool maybe) and he also told to the poor boy that he has a bomb in his head that will explode if he will talk with anyone.
Step by step the girl try to convince the boy to tell her the story. (hide spoiler)]

[Patrizia]

Amy wrote: "Might be The Only Alien on the Planet"

Yes, it is! Thank you so much :)
The italian title is "Smitty".