Half Brother by Kenneth Oppel

Ben Tomlin was an only child for thirteen years. Then his parents brought home a baby chimp.

It isn’t long before Ben is Zan’s favourite, and Ben starts to see Zan as more than just an experiment. His father disagrees. Soon Ben is forced to make a critical choice between what he is told to believe and what he knows to be true — between obeying his father or protecting his brother from an unimaginable fate.