Alan, as Kevin says, the ought sizes are more or less Martin's standards for smaller bodied guitars - single ought is a parlor size as was played around the turn of the century (Joan Baez played one), double ought are slightly larger and are played by some blues and fingerstyle people, triple ought and the similar OM are very popular "small" guitars favored by all kinds of player - Clapton on Unplugged, Rory Block - just lots of people. Jumbo, Dreadnaught and 0000 are larger. The important thing is that these are all steel string. Here is a picture of an 0, 000 and OM (the twelve string) for comparison - there is also a good size chart at Martin's site
Most nylon string guitars are very close to 00 in size - the one that I built fits that case perfectly. However there are some big differences between classical guitars and steel string - you might want to make sure your son really wants one. Mine did, so I built him a kit from LMI. Here is the build thread
http://www.kitguitarsforum.com/archives ... 82_0_9_0_C
http://www.kitguitarsforum.com/archives ... 86_0_6_0_C
And here is the kit wizard http://www.lmii.com/CartTwo/GuitarKitWelcome.asp
I would also serious suggest getting Cumpiano's book , it covers building a classical as well as steel string.