Max's early childhood was pretty messed up, to say the least. He witnessed his father, "Killer" Kane, strangle his mother to death, a traumatic event that resurfaces later on in the novel when Kane escapes from prison and abducts his son.
As a result of this terrible event, Max endured an early childhood notably lacking in love, attention, or any outward displays of affection, all the things that are important for a child in their formative years.
Max's grandparents would subsequently come to take care of him. But so much psychological damage had already been done by then that Max was still a very troubled young boy. After seeing his father murder his mother, he became almost allergic to the thought of anyone touching him, so much so that he'd kick anyone who did.
This applied to anyone who hugged him. Max's aversion to hugging can be seen as an extreme reaction to what his father did to his mother. Hugging may be very different from strangulation, but to a traumatized child like Max, it's just another invasive physical action.
In addition, Max is not used to hugs because of his loveless childhood, so he instinctively recoils from what to him is an unfamiliar experience.