You are having a RAM problem. I will explain.
Apple has never wanted to beat a dead horse over this issue since most of us use the desktop as a garbage heap.
It is the easy / lazy way out. Mea culpa.
Apple provided us with (default) ƒolders to store --> Documents, Applications, Movies, etc.
Still too difficult for many.
Why is this an issue?
Because every item on the desktop REQUIRES a RAM allocation.
You can either suggest to your son that he follow my advice, or you can load up the old Mac with (maximum) RAM.
The desktop obviously is a ƒolder within your Users ƒolder.
BUT, it has one special characteristic that no other ƒolder has.
OS X allocates RAM to every file, folder, icon, etc. that exists there.
To free up RAM and to speed up boot time (disabling extraneous fonts helps a lot too) and to speed up Finder operations in general, create a ƒolder at Root Level on the boot volume of your Mac's Hard Drive. Name it "Desktop Detritis" or whatever you wish. Move almost everything from your desktop there, and move applications to the Applications ƒolder. Naturally, do not attempt to move the icons for your Hard Drive(s). Then drag that folder to the right side of your Dock. Voila.
There is a very good book called "Degunking Tiger" that describes this and many many more tips like it.
Also, I suggest using xSlimmer to regain a ton of space on your hard drive. Also I use (either) "Omni Disc Sweeper" 1.5.2 or "WhatSize" 10.3.92 to show me the HUGE invisible files that are clogging my Macs, e.g. huge Swap Files. If you use Adobe applications or do heavy duty work in Desktop Publishing, Photographic work, Graphic Images, MIDI, etc. you may have invisible Scratch Disks of a Gigabyte or two in size. I also use FileBuddy9 or TidyUp! to search out duplicate files and delete them.
