I will repeat. The concern here is based on parental child abduction. There is an international treaty on this issue, known as The Hague Convention on Child Abduction.
This does not come into play here. It is extremely unlikely that preventive measures ( I.e. border protection intervention) would be applied to a 17-year-old boy. He may be months away from his 18th birthday, when he is considered an adult and child custody rules no longer apply. IF ( and I seriously doubt there would be) there are concerns at the airport, they would first address the son., who could easily resolve any concerns. The parents could also show that they have return tickets for all three.
The protective rules are good and appropriate for younger children, who unfortunately can become pawns in a parental custody dispute. I have seen some heart-wrenching cases of that, but all involving children well under ten.
And when people are traveling with nieces or nephews, it is always important to have the consent letter, for many reasons. But a 17- year- old traveling with his mom and step-dad? Not an issue.