The PC is for gaming only. It should be able to run Skyrim with mods on Ultra. Upgrading is not that impotent. Same goes for over clocking. We are going to use all the standard clocks.
Please notice that I live in Switzerland an that all the cool deals don't work with me. PC Part picker is fine, as prices are more or less the same here.
The 1100$ go with the OS so about 1000$ for hardware.
$780-$850 (depending if your country qualifies for rebates)
http://pcpartpicker.com/user/jra/saved/mxLwrH
Add another $200-$300 if you want the R9 290/290X. I assume he'll be gaming at 1080p, correct? The AMD 280 should be more than be capable of Ultra settings at 1080p.
And run those two SSD's in RAID 0 for ultimate performance. He'll have half a terabyte of SSD storage, which is more than enough to start with. If he needs more, he can always add a cheap hard drive for $60 later on.
GPU: Sapphire R9 290 (Tri-X, Vapor-X), MSI Twin Frozr. If the build ends up being too pricy, you can go down to a R9 280X and still have great performance.
You will need to shop around, but this has a GTX 770 4gb (moded skyrim will use it all (or close)), a xeon e3 1231v3 and a reliable but cheap motherboard. You will need to update the bios, but you can use the CPU to do that.. if you are nervous, the 1230v3 is still a great choice. This has a 250gb SSD for OS and skyrim (ssd really helps with load jitters (load and you go in skyrim) and a 1tb HDD for backup/media.
This also has a fully modular PSU and a small form factor case with excellent airflow.