回複：but 824 is only for kid who is not in usa while

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.