Christopher has some fairly normal reactions to his mother's death, and many more abnormal reactions - at least according to how most people would react. What is "normal" and "abnormal" is incredibly subjective. People grieve very differently.

What we might consider normal are the number of questions that Christopher asks about his mother's health and death. In Ch. 53 we see him really struggling to understand how she could have died.

"Mother was only 38 years old and heart attacks usually happen to older people, and mother was very active and rode a bicycle and ate food which was healthy and high in fiber and low in saturated fat like chicken and vegetables and muesli" (Haddon).

These are perfectly natural questions to be asking because his mother was not visibly sick prior to her "death." Then, he didn't see her in the hospital and he didn't get to go to a funeral, so he only saw his mother healthy and then he didn't see her at all. His constant questioning is normal. Unfortunately, his father does not handle these questions well.

Father said that he didn't know what kind of heart attack she had and now wasn't the moment to be asking questions like that." (Haddon).

If now was not the time, when would be?

As for what could be considered abnormal, we never hear about Christopher actually crying over his mother's death, expressing that he misses her, saying that he loved her, or wishing he could see her again. All of these would be considered "normal" reactions by a son who suddenly lost his mother. That being said, we do know that Christopher doesn't understand or express emotions in the way that most others do. We see this from the very beginning of the book when he explains that Siobhan had to show him what different faces and emotions were so he could begin to learn how to identify them when he saw people reacting a certain way (Ch.3). 

To say that Christopher feels nothing would be incorrect. He does have feelings and emotions. He simply is incapable of expressing them in ways that we would consider "normal." Many times in the book when Christopher is trying to solve a problem that bothers him, a problem about his family or about Wellington, he reverts to a math or science problem in the very next chapter. This shows that he is concerned and feeling worry or sadness or stress, but his way of dealing with it is to try and work it out in a more logical way through the subjects he DOES understand. So, in dealing with his mother's death, he may not comment on missing or loving her, but he clearly shows interest in what happened to her.