Building a PC is pretty much a cross between Meccano and Lego its pretty difficult to go wrong as long as you apply common sense. On that basis I think the learning opportunities from assembling the parts are pretty slim but if your son does build his own PC he should get a good confidence boost and a sense of satisfaction. More of the learning would probably come prior to the build process from researching what parts are needed and why some are better than others and what makes them better as well as after the build should your son want to dabble in overclocking (good idea to find this out prior to buying parts for him). It isn't really the case anymore that you can build your own home PC for less than you can get one off the shelf, most self builders tend to be enthusiasts who know what parts they want to go into their machines either for aesthetics or maximum performance (or both).
For the base unit (not including monitor / mouse / keyboard) you will need;
Case
Power Supply Unit
Motherboard
Memory
CPU
CPU Cooler
Graphics Card
Hard Disk Drive
Optical Disk Drive
Windows License
If you’re on a budget then look at a build around an AMD CPU but the above is going to cost a pretty penny, certainly more than a basic £300 PCWorld machine.
If you want further advice then overclockers.co.uk have a forum where you could post what it is you want to achieve along with your budget and folks there will help you select the appropriate parts.
For the base unit (not including monitor / mouse / keyboard) you will need;
Case
Power Supply Unit
Motherboard
Memory
CPU
CPU Cooler
Graphics Card
Hard Disk Drive
Optical Disk Drive
Windows License
If you’re on a budget then look at a build around an AMD CPU but the above is going to cost a pretty penny, certainly more than a basic £300 PCWorld machine.
If you want further advice then overclockers.co.uk have a forum where you could post what it is you want to achieve along with your budget and folks there will help you select the appropriate parts.