Why do you want to have a rule about a mother coming with her son and being present in the exam room.
In the past it was always the common rule (or practice if you prefer) until at least 16 age, and it was not to follow an explicit doctor rule but rather to respect common use but it did not mean that the mother saw always her son naked because thre was each year at least in France a mandatory check up physical examination at school, a private doctor had not tpoo perform such an exam, we went to see him rather for an illness, so it was different sometimes we had to undress until briefs, sometimes to strip complrtely and sometimers to be only bare chest.
In my opinion, it must depending on boys ( some arez more modest or shy toxards nudity than others, some wants the presence of their mother because they are anxious at a private doctor's office and so on ..; ) Anyway it cannot be a big issue if a mother see her son in the nude at least until 16 age.
It is like those who prefer to continue to see their pediatrician until the end of the college years about 21 age rather that at the end of high school years or even 16 age, it does not matter , why would it be different for a college student to be examinede by a pediatrician than by a general doctor ?
because they are acting differently? in which way ? the undressing ? A pediatrician requires more frequently the complete nudity than a general doctor ? Even if it is true, why would it be important if the young man is used to see that pediatrician and prefers to be examined by him?