Agree with the others. Superfudge is fine. Even if your son does believe in Santa at this age (he may not, some children hide the fact that they “know” from their parents), I can’t imagine him entering 5th grade still believing.
I used to teach in a small private Catholic school.
6 out of 7 of my homeroom 6th graders believed.
The one respected the fact that he would not disclose to the others.
In the 7th grade of this school, only one or two of the girls believed. Again, the other students did not make fun of the girls or disclose the truth.