824 can be for anyone, even principal applicant who first wished to AOS in USA when he sent his I-140, then change mind to CP and sent I-824 instead I-485.
In his case, if his son is elligible for following-to-join in terms of age, his son can select either I-485 like he did or I-824 if wish to CP. Again, his son's immigration age should be less than 21 years old to be qualified to this selection.