If you're not opposed to riding an alloy framed bike, I can suggest a 4th that may very well suit your sons small size. Dawes Vantage!
I'm 5'3" and ride the 48cm/19" model and theres more than enough room on it for me. Theres a 43cm/17" version also.
The same bike was used by one of the guys in the bike touring club I'm with and he used his for a 1000 mile trip from Lands End to John O'Groats sometime last year and said he had no problems with it whatsoever.
The bike comes with a full set of braze ons, a fairly sturdy rear rack and mudguards. Vittoria Randonneur tyres have so far been pretty much bomb proof.
My one problem with mine was the short stem and stupidly sharp handling which I've since calmed down with a longer 110mm stem and inline seatpost. Its a wonderful ride.
Just a thought