I'd avoid dragons unless you have some serious space available. Beardies will grow to over a foot (including tail) and water dragons can clear 2 feet (again I include tail).
If you can provide the room, sure! Go for it! Dragons, imo, have the most personality of any pet reptile (some might say iguanas are affectionate but they're also often very bitchy). They also have a very mild temperament and are great for handling.
Few questions to ask yourself with reptile selection:
Nocturnal or diurnal?
Handleable or display?
Big or small?
It also depends a lot on how old your son is. I love beardies but you really need to supply them with fresh veggies every day and juicy bugs as well. It can get to be a bit of a hassle if you aren't deadset on it (and bear in mind that unless your son is over 15, *you* end up taking care of the lizard
)
I'm sure googling for a caresheet will yield more results than you care to read... It's all pretty standard so you ought to be able to trust what you find. When you've decided on the species and go to pick up a book, get one by Philippe DeVosjoli - he writes "the book" on pretty well every common captive reptile species and is among the most respected authors.
Very good beginner lizzies:
Beardies
Water dragons
Leopard geckos (nocturnal)
Anoles (not really handleable but simple and cheap)
Veiled chameleons (can be crabby, but sturdy as a rock if you can provide them the large amounts of calcium and water that they need)
Blue Tongued Skink (We've had one in the store for almost a year now.. No takers, but it's probably the most amusing lizard I've ever seen. He likes to be hand fed grapes)
Bad choices, imo:
Day geckos (too jumpy.. I have one)
Tokay geckos (they would try to eat your cat)
Green iguanas (too large and temperamental)
Monitors (get quite large and many species are aggressive - I think the Savannahs are kind of sweet though)
HTH - let me know if there's anything else I might be able to tell you