form I-539 signature

Hi, guys:
I just got the “I-797C, notice of action” for my son’s form I-539 (H-4
visa extension). But it was rejected, because there was not signature of the
applicant. In fact, I had signed the form in the line “signature of person
preparing form”. I do not understand why does my son still need to sign
the form since he is only 5 years old?

Thanks.