SOLVED. Book about a boy who talks to his brother who died when he was a child and escapes from institution. [s]

[Lucy]

It's about a boy, maybe 17/18 who's spent his teen years in an institution because he talks to his brother who died when he was a kid. His brother lives inside his head and whenever he talks in the book it is in italics. He is also still seven years old, the age he was when he died.
The story is about the boy running away from the institution because the psychiatrists are getting close to making his brother disappear. He doesn't know how to survive in the outside world though. His prized possessions are these model aeroplanes that he and his brother used to make as kids.
The story might be about him (or them) trying to find their father. He used to send letters to them as kids saying he was going into outer space, until the letters stopped one day.

[Ayshe]

We Used to Be Kings by Stewart Foster maybe?

[Lucy]

Ayshe wrote: "We Used to Be Kings by Stewart Foster maybe?"

I have no words to tell you how thankful I am for this! I am buying this book right now, no matter the cost. Thank you so much!