The Hat Diaries by Nadine Haruni

Ryan Rigbee is a thirteen year old loner who is suffering from losing his father. To cope, Ryan takes imaginary adventures with his dog Shadow where he can be anything he wants in any part of the world. But then he meets Merlo and his life changes forever. Merlo makes these adventures become real through wearing a series of magical hats. These adventures help Ryan discover more about himself and to grow in confidence. Eventually, the imaginary world and the "real" world become intertwined, building to an incredible climatic finale. A superb fantasy adventure for anyone looking to escape!
This story will help those struggling with self-assurance, it is a great book for all ages but I recommend The Hat Diaries primarily for ages 8-12.