Callum appears as a character in The Enemy.
He is part of the Waitrose crew and sees Waitrose as the only safe place in London, noted when he doesn't want to leave for Buckingham Palace.
When Small Sam returns and sees that everyone else is gone except for Callum, he pumps up a tire for Small Sam's flat-tired bike.
Later in the book while he is in the shop, grown-ups come in while he listens to music. The music is by ABBA, a band his mom listened to with him before her death. He also states he killed his mom by suffocating her with a pillow. It is unknown if it was before the disease or when the disease came around.
Callum sets up a running track around Waitrose to pass the time. At first he loves his new life but then realises he is crying as he is lonely.
Callum sees St George and his followers attacking and tying to break into Waitrose. He figures if he leaves them alone then they will just go away like the rest of the grown-ups.
He has such a large fear of being outside that he chooses to not run away and stays in his seat. When he sees the grown-ups, he stays, stating to himself going outside would be worse. This ends in his life.
In a later chapter it is stated that the grown-ups who attacked him put his head on a spike in front of Waitrose. This is mentioned in The Fear, as Shadowman sees it happen.