Okay, I'd say first thing to do is to make sure that there are drivers for Windows 8 for your system devices and peripherals, etc. If there's a Microsoft Windows 8 compatibility wizard like they had for Windows 7, then download and run it. If not, then use the Windows 7 compability wizard, I have a feeling that they're probably pretty close.
For Windows 7, they recommended a minimum of 2GB of memory, so if you're aiming for Windows 8 I'd upgrade the amount of memory to 4GB if you can. Memory is cheap, so that won't cost much and will make the difference between a dog and a system that might run okay. Also, I found that I couldn't install Window 7 on my Windows XP system...it passed the Microsoft Windows 7 compatibility tester, but when I went to the Dell website, they didn't have drivers for Windows 7 for it. Which was a bummer...I could have gotten around it by putting in a graphics card and a sound card that did have Windows 7 drivers, but I took that as a message. I re-installed Windows XP on the system, and it runs really well now, having gotten rid of about 8 years of cruft. So be careful before committing to Windows 8 that the system is compatible with it, and that you have enough memory and the right drivers.
One thing to consider is doing what I did, re-installing Windows XP, which really helped bring my desktop system back to life. You could also pick a Linux distribution, and your son could learn about Linux and how to use it as a desktop system. A lot of good learning to do there, and the apps, most of them free, are out there that will make a Linux system useful as a desktop. I'd still add memory, again, because memory is cheap, if you can. That, along with adding an SSD are a couple of the best performance kicks that you can get for a computer. Adding an SSD in this case, is most likely not a good move, since SSD's are not well-supported until you get to Windows 7, and we're talking about what you should do if you can't install Windows 7 or 8.
Anyways, those are my thoughts about it. Remember that as time goes on, new OS's may seem to get faster, but they don't get faster on older hardware, they get faster because the hardware that they're distributed on is newer and faster. In fact, the reverse is true, newer OS's are bigger and slower on older hardware, that's just life.
Have fun with it, in any case. You might add some memory and go the Linux route, or maybe you can wedge Windows 7 or Windows 8 on the computer, but don't expect either to "revitalize" old hardware. The best revitalization would come from either one of the smaller Linux distributions or a re-installation of Windows XP, in my opinion.