Paying to upgrade your son, or downgrading yourself are the morally right options IMHO. Considering that it's just a 1:15 flight, why not treat your son's seatneighbor to a nice surprise, offering to swap for ET for CE?
Surprised they even managed to book him in a different class. I too would have thought that they'd consider your son to be an unaccompanied minor in this case and therefore require the Skyflyer Solo Service.
Certainly BA.com doesn't allow to book "less than 1" Adult....