Since he cannot be their derivative at twenty-two, his application would be independent. Sequencing the parents first could let the family evaluate the process before paying a second nonrefundable fee and gift, but he should re-check the current official terms before deciding to wait.
Independent applications can be sequenced
Because the son's route, if any, is as a principal applicant, his timing does not depend on his parents' file, and the family can sequence rather than duplicate. Hypothetical example: the parents proceed first, learn from the vetting stage what evidence is requested and how the connected employment-based category is applied, and only then decide whether the son's own eligibility and admissibility justify a second processing fee and gift.
During that interval the son should assemble his own admissibility and eligibility documents and consider whether a temporary category such as a student status or, if he has a listed profession, TN, meets his needs in the meantime; those are separate applications with their own requirements. Before either application, the family should print and date the official program page, confirm the fee, the gift amount and the stage at which the gift is due, and re-read the page on the day of each payment, since the terms are published on an official site and may be revised.