This model of PowerBook (1.67Ghz Al) can be upgraded to Mac OS X 10.5.
There is some advantages to upgrading and some disadvantage. One of the disadvantages is the ability to sync iPhones running iOS 6 (Which requires a newer version of iTunes that runs on Intel Macs)
Now I use a PowerBook G4 1.67 17" running Mac OS X 10.5.8 and does all I require. The only thing it can't do that well is play back flash websites. (Like Youtube) The Java is out of date, but the Camino web brower still handles most website very well indeed. (There are applications like YouView that will play back YouTube Videos very well) As this PowerBook is 9/10 years old, it has some web restrictions, as the web technologies have moved on.
This big disadvantage is the cost of upgrading. The Retail DVD's for Mac OS X 10.5 is expensive (As they are obsolete) Memory and hard drives are very reasonable and can be cheap to upgrade.
The thing to think about when making this discussion is, does your son require a machine that can run the latest version of software like iTunes? Does he require flash video play back in a fast web browser?
If he is happy with using the PowerBook to do basic web browsing, office work and maybe use some older PPC based applications, then it's ideal.
Hope this helps.
Here's the requirements for the iPhone 4s.
Mac OS X 10.5 Leopard installation system requirements
Look on eBay or Google the installer part numbers to possibly find at an on-line store. Here's what to look for:
MB427Z/A Leopard 10.5.1 install DVD
MB576Z/A Leopard 10.5.4 install DVD
MB021Z/A Leopard 10.5.6 install DVD (single user)
MB022Z/A Leopard 10.5.6 install DVD (5-user family pack)
Installing Mac OS X 10.5 Leopard
Mac OS X 10.5 Leopard Installation and Setup Guide
After you install the base 10.5, download & install the 10.5.8 combo update at http://support.apple.com/downloads/Mac_OS_X_10_5_8_Combo_Update
The DVD should look like this
Caution - Leopard does not support classic mode. So, if you currently open OS 9 apps in classic mode, you won't be able to do this if you upgrade to Leopard.