This would be mine:
Still £90 to spare, personally I don’t think the i5 skylake is worth the upgrade, you’d be jumping up to DDR4 RAM too, which costs more than DDR3, not drastically, but enough. Not enough improvement on the 4690k compared to the 6600k. If you want to see for yourself here’s a pretty good comparison chart: http://download.intel.com/support/processors/corei5/sb/intelcorei5comparisonchartrev9.pdf
As for the £90, you could save it if you want, you could put more money into a bigger and better case, you could upgrade the SSD to a better one, could throw in an optical drive if you want one, fuck it he could throw in a decent closed loop water cooling kit (he couldn’t do all of these of course). As you say, he’s 15, I imagine that £90 is quite a lot of money to him still, so maybe it’s worth mentioning he can do it a little bit cheaper, or he can spend absolutely all the £800, but he’s not really going to see much of a performance boost.
I’m a little biased against DDR4 RAM atm, it’s coming out, and it’s not going to be all that much difference. Frequency has increased sure, but the RAM is still a decent physical distance away from the CPU, which is where the main bottleneck with RAM lies. As I say, I’ve upgraded to DDR4 and seen no drastic or noticeable improvements.