E5N1 now has a British passport to go with his U.S. one. This will make things slightly easier next time we visit - and significantly easier if employment or immigration issues force us back to the U.K. more permanently. This is his latest passport photo - fortunately they relax the prohibition on smiling for under six year-olds!
His passport application hit a back-log at the British Embassy in Washington D.C., so it has taken a while. Not that we can go anywhere at the moment, with the rest of us subject to pending visa-renewal applications.
So here he is - the newly documented British/American dual citizen - he can legally live and work (well OK, live) in any number of places in North America and Europe. Well, just look at that picture, what country could turn him away?