Edit, I tried posting amazon links and that didn't work, I will type in the exact computer I put together this summer, the items are from Amazon.
ROSEWILL ATX Mid Tower Gaming Computer Case, Gaming Case with Blue LED for Desktop / PC and 3 Case Fans Pre-Installed, Front I/O Access Ports (CHALLENGER S)
Corsair CX Series 750 Watt 80 Plus Bronze Certified Modular Power Supply (CP-9020061-NA)
Intel 7th Gen Intel Core Desktop Processor i7-7700K (BX80677I77700K)
MSI Enthusiastic Gaming Intel Z270 DDR4 VR Ready HDMI USB 3 ATX Motherboard (Z270 GAMING M3)
Cooler Master Hyper 212 EVO RR-212E-20PK-R2 CPU Cooler with 120mm PWM Fan
Corsair Vengeance LPX 16GB 2x8 GB DDR4 DRAM 3000MHz C15 Desktop Memory Kit - Black (CMK16GX4M2B3000C15)
As far as hard drive, depends if he wants a SSD hard drive or regular, the SSD hard drive loads in a couple of seconds and is very fast, but does cost more.
the memory I posted for some reason is really high, I paid like $135 for mine, I believe I read that the memory prices and video card prices are real high right now for some reason.
You will need a video card as well, I have a Zotac Nvidia 1080 card, which is around $500 or so maybe more, but you can go cheaper on one if he is just doing regular gaming. I do the Virtual Reality stuff and you have to have really quick stuff to run it.