While I drill deeply into certain issues (a rather few issues) and the details in related information, I really am NO expert. And I am especially NOT qualified to offer personal advice.
I cannot say what is "correct" in an individual's actual case, other than to reiterate the importance of honestly, as accurately as practically possible, reporting facts responsive to questions in an application or asked in an examination.
I will offer some further observations (as I am wont to do):
The Arithmetic:
The arithmetic is hugely important, but it is basically automatic. It is just counting days and doing some very basic addition or subtraction. It derives from and thus definitively depends on travel history dates.
For a PR card application, or a PR TD application, the counting of days outside Canada is automatically computed by the interactive pdf form. When, in Section 5.5, the PR accurately enters all dates the PR exited Canada, the 'From' date, and the next entry into Canada, the 'To' date, the form automatically computes the number of days outside Canada for that trip, and also automatically adds the total for all the trips listed.
IMM 5444 also computes and will separately total the number of days outside Canada for which the PR listed a reason for the absence that qualifies for credit toward the RO. For a dependent-child-PR accompanying a Canadian citizen parent, this is reason "B" (which does not exclude the child-PR having other reasons for being abroad, like attending school). It will further compute and show the total number of days spent outside Canada excluding absences qualifying for a credit despite being abroad.
Note: The application form can be used to compute days outside Canada based on theoretical travel history dates, including future dates. Example, in a draft version one can enter period to be assessed as 2019-03-03 to 2024-03-03, and list a trip in the travel history table for 'From' 2022-07-11 and 'To' 2024-01-01 and in the 'Days' column it will automatically show that period of time to be 538 days outside Canada. If reason 'B' is checked in the reason column, those days will be in the total the form shows for "total time spent outside Canada," but not in what the form shows for "total time spent outside Canada excluding reasons A, B, or C"
Generally, if the "total time spent outside Canada excluding reasons A, B, or C" is less than 1095 days, the PR has met the RO.
HOWEVER, as cautioned, if that is in part periods abroad for which the PR claims a credit (like here, entering 'B' in the reason column for trip accompanying citizen parent), and the total time outside Canada altogether, is NOT less than 1095 days, RO compliance is dependent on the validity of the credit for days abroad, which is an EXCEPTION, and it warrants remembering that getting credit for an exception can involve more stringent scrutiny. It definitely requires providing additional information and supporting documents.
Where the child-PR continues to be abroad accompanying a citizen parent, after the PR's 22nd birthday, that poses a bit of a wrinkle. You have indicated this will be the likely situation with your son. As of his 22nd birthday he will no longer qualify for the accompanying citizen parent credit. So, if he is already abroad in the days prior to his 22nd birthday and continues to be abroad following that day, since there is just the one continuing trip or absence, there is no way to separately enter the before and after days in the Section 5.5 table. Listing those periods as separate trips is not accurate, since all periods of absence, all trips abroad, are to be listed ONLY by the date of exit from Canada and next date of entry into Canada.
There are multiple ways to approach that scenario, and which one is employed is really the individual's choice, which should be OK as long as, one way or another, the information submitted accurately communicates the absence was continuing and did not involve a return to Canada (note that all 'From' and 'To' dates listed in travel history table indicate the PR was in Canada that day, so no 'From' or 'To' date should be put into the travel history unless the PR was actually in Canada that day, either leaving or entering). Thus, for example a Supplemental-Explanation (some here refer to this as a LOE, or letter-of-explanation) might be submitted with the application, explaining things, in which case it would probably be prudent to at least reference there is a supplemental explanation in box in the table for stating other reasons . . . perhaps "see supplemental explanation regarding days after 22nd birthday," and the supplemental explanation itself could state something like the PR continued to be abroad accompanying his citizen parent and to complete a course of studies in college. Keeping it simple, simple and honest.
REMINDER: PRs abroad should be sure to keep up with potential changes in the rules, even potentially in the law itself.
