You can go buy a whole plane full of tickets....and you will have to have them ticketed in names that have valid ID’s.
You cannot then give away those seats/tickets to whomever you choose....just because you paid for those tickets.
If those listed as the ORIGINAL ticket holder (in this case, the 18 year old son) don’t show for the flight, you cannot decide who gets to sit in those seats, just because you paid for the tickets.
It’s clearly listed on airline tickets....ONLY named passenger, for EACH ticket, may use the ticket.
That ticket is assigned to a particular seat, right? How many stories do we hear of a First Class passenger swapping seats with one of our military members? This shouldn't happen according to your logic, as no one but the person who paid for that seat, and is assigned to that seat, can use that seat.