UA is not going to risk a (I believe) $10,000 fine flying Jr. to London without a Passport. Not to mention, the return trip. UA does not know who is holding the Passport.
I believe the $10,000 fine for the carrier was correct a few years ago. Flying without passport. Not to mention the carrier (UA in this case) must fly Jr. back to the USA - with a UA agent staying with him, until departure back to the USA.
Perhaps times have changed?
I'm with UA on this one, especially if Jr. is checking bags.