Go with the Sibley Guide to EASTERN BIRDS.....its "close" to the Sibley Guide to Birds but its in a smaller format and also only covers the Eastern U.S.....as your son starts out he doesn't need the Western Birds, many of which have an Eastern counterpart that can be very similar, in the smae Guide....for practical purposes the Eastern Guide is the same book, with a couple less illustrations per bird, in a smaller format. The text is updated, (2002 printing as compared to 2000 on the Guide to Birds), and is more "accurate" than the earlier version.....
Since you're in South Florida I'd also recommend a couple of STATE sepcific Guides.....Birds of Florida, (one of the Smithsonian Handbook series), by Alsop, (whose artwork rivals Sibley). Its specific to Florida and includes more "information" about the individual birds than the Sibley Guide....
I'd also get a copy of FLORIDA BIRDS by Maeher and Kale.....this one isn't as comprehensive as the Smithsonian Guide but it has good coverage of the many Exotics that you un into in South Florida, (something that Sibley all but ignores).
Round out his beginners library with a couple of other Sibley books: Birding Basics--well worth the read as it "encourages" the reader to be an OBSERVER and not just a "lister", (nothing wong with that but I've always thought that if you were watching birds you should be interested in knowing more about it than just that you saw it); and HAWKS IN FLIGHT....worthwhile for its "expanded" information on a group of birds that is highly attractive to beginners because they are large and more easily "observed" than the little stuff....
I'd encourage you to not concentrate on birds alone....get him into the "full meal deal" so that he is looking into not just birds but mammals, reptiles, amphibians, flowers, butterflies, etc.etc., which will encourage him to be more likely to be interested in "interactions", and "cuase and effect", than he will be if he just concentrates on birds...
Be careful....you get him into i.d.'ing those little "brown disappearing birds", and he'll start getting interrested in the Environment beyond the species that he hunts. Sure as shooting as soon as that happens people will start calling him "Bunny Hugger" and you don't want that cause we all know that ALL of those folks are not only ANTI-HUNTING but want our guns as well....
Steve