No, Kevin's dad is not really a magician. What you are referring to in your question is a cruel comment that Loretta makes about Kevin's dad while both Max and Kevin visit Loretta in the tenements in order to return her purse to her.  Loretta remembers Kevin and his mother, Gwen. As Loretta shares her memories, she also shares the memories of Kevin's dad. Specifically, Loretta shares that “as soon as he heard the magic words ‘birth defect,’ he disappeared.” Because disappearance is often the climax of a magician's act, Loretta claims that Kevin's dad's disappearance due to his son's birth defect of Morquio Syndrome is the climax of his act as well. As a testament to Kevin's positive character, he clings to the idea that his dad is truly a magician (like Merlin from Kevin's favorite legend of King Arthur). There is also a neat piece of irony about this comparison by Loretta.  Even though Loretta is very cruel in first meeting the boys, it is Loretta who saves Max's life when he is kidnapped by his father. In this way, Loretta risks her life to save Max.