I would think something like this would look great and last a few years.
YOUR BASKET
1 x
Sapphire Radeon R9 270X Boost OC 2048MB GDDR5 PCI-Express Card £129.95
1 x
Gigabyte Z97P-D3 Intel Z97 (Socket 1150) DDR3 ATX Motherboard £62.99
1 x
NZXT Source 340 Midi-Tower Case- Black Window £59.99
1 x
Corsair Hydro H75 Performance Liquid CPU Cooler £59.00
1 x
Intel Pentium K Anniversary G3258 Socket LGA1150 Processor - Retail £55.99
1 x
TeamGroup Vulcan GOLD 8GB (2x4GB) DDR3 PC3-17100C11 2133MHz Dual Channel Kit (TLYED38G2133HC11ADC01) £50.99
1 x
Kingston 120GB SSDNow V300 Drive SATA 6Gb/s 3 2.5" (7mm height) Solid State Hard Drive - (SV300S37A/120G) £46.99
1 x
Antec VP450P 450W 85% Efficient Continuous Power Supply £38.99
1 x
Seagate Barracuda 500GB 7200RPM SATA 6Gb/s 16MB Cache - OEM (ST500DM002) HDD £36.95
1 x
NZXT CB-LED10-WT 12x White LED Sleeve - 1m £9.98
Total : £566.82 (includes shipping : £12.50).
I would recommend using the Windows 10 test as your OS, it is not too bad and free for a while, my kids like it more than W7. But load it onto USB if possible.
My boys are 9 and 11 now, they have been using computers since around 3 sitting on me or the wifes lap using Cbeebies n such. They don't use optic drives.
I would recommend taking the above case, and mounting the water cooler in the rear with the two case fans moved to the front. The LED light will make it pop nicely as it highlights the internals.
I would also recomend an SSD, my kids manage with a 120gb and 500gb HDD. No storage issues as kids do not usually hoard data on PC's. The SSD makes a huge difference, honestly! Even if you just have to save money, get a single SSD and add an HDD later.
The system, being Z97, is upgradable, but for a childs system, well he will be using a 1080p monitor I would guess, an Nvidia 750ti or AMD 270X would be plenty for medium/high settings most games. AMD 280 and AMD 285's well some are not really clocked any higher than some of the 270X if I remember, the more you look at an R9 290 which adds an extra £100 to the bill, you are also looking at around 550w+ for your psu with the right connection and a decent 12v rail, maybe something like the £55 Antec gamer or such.
And don't forget, it may get him interested in learning about PC's, it would be a bit of a showcase in his room.
I would also advocate the purchase of some periphials that will liven up the experience and add some color.
I have been using Overclockers larger mouse mat for years, it is still going strong, my keyboard and mouse sit on it it's that big, the one I have listed is smaller, my kids have my old Ratz Cyborg mousematt which is about the size of the one listed.
The keyboard and mouse I plan to purchase, my kids have already went through two keyboards, but now they are older and a bit more carefull.
The keyboard and mouse have blue LED lighting, my kids think it looks cool, if they told me otherwise I would think they were after extra pocket money or had drank all the milk again.
Headset, well whether it is a consol or PC, kids and headsets, talking with friends, and not irritating your listening pleasure with tinny PC speakers screaching from the room.
YOUR BASKET
1 x
CM Storm Devastator Gaming Bundle - USB Keyboard & Mouse £29.99
1 x
SteelSeries 4H Gaming Headset - Black (61022) £19.99
1 x
OcUK Mega Mat Medium Elite Tactical Gaming Surface £9.95
Total : £69.53 (includes shipping : £8.00).
Only my recommendations. Currently my own kids use one of my older PC's, micro atx within a Silverstone TJ08B-E case, which sits in a PC cabinet within their computer desk.