If you have need for a dual core CPU, you have need for a quad core CPU. Future applications and games will be much more threaded, and if he runs a home server, especially, he needs the ability to multitask. The quad core is always the better bet.
EDIT: If it was me, I would build one myself and save some money, plus your son can likely assemble the PC if he is very mechanically/electronically inclined at all. It could be a fun family project.
I would build something like this:
PCPartPicker part list:
http://pcpartpicker.com/p/13HU1
Price breakdown by merchant: http://pcpartpicker.com/p/13HU1/by_merchant/
Benchmarks: http://pcpartpicker.com/p/13HU1/benchmarks/
CPU: AMD FX-6300 3.5GHz 6-Core Processor ($119.99 @ SuperBiiz)
CPU Cooler: Cooler Master Hyper 212 EVO 82.9 CFM Sleeve Bearing CPU Cooler ($29.99 @ Newegg)
Motherboard: Asus M5A97 LE R2.0 ATX AM3+ Motherboard ($68.99 @ Newegg)
Memory: Corsair Vengeance 4GB (2 x 2GB) DDR3-1600 Memory ($36.15 @ NCIX US)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($64.98 @ Outlet PC)
Video Card: PowerColor Radeon HD 7870 XT 2GB Video Card ($209.99 @ Newegg)
Case: Cooler Master Elite 430 ATX Mid Tower Case ($29.99 @ Newegg)
Power Supply: Corsair Builder 500W 80 PLUS Bronze Certified ATX12V Power Supply ($29.99 @ Newegg)
Optical Drive: Asus DRW-24B1ST/BLK/B/AS DVD/CD Writer ($16.99 @ Newegg)
Operating System: Microsoft Windows 7 Home Premium SP1 (OEM) (64-bit) ($89.98 @ Outlet PC)
Monitor: Acer G215HVBbd 60Hz 21.5" Monitor ($89.99 @ Microcenter)
Total: $787.03
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-06-06 17:10 EDT-0400)
Now, that PC would need a mouse and keyboard...but has everything else, also the CPU and GPU for this one are considerably more potent than the solutions you're likely looking at. Which means his gaming and overall system performance would be better.
PCPartPicker part list:
http://pcpartpicker.com/p/13HZO
Price breakdown by merchant: http://pcpartpicker.com/p/13HZO/by_merchant/
Benchmarks: http://pcpartpicker.com/p/13HZO/benchmarks/
CPU: AMD A10-6800K 4.1GHz Quad-Core Processor ($149.99 @ Newegg)
CPU Cooler: Cooler Master Hyper 212 EVO 82.9 CFM Sleeve Bearing CPU Cooler ($29.99 @ Newegg)
Motherboard: Gigabyte GA-F2A85X-UP4 ATX FM2 Motherboard ($107.99 @ Amazon)
Memory: Corsair Vengeance 8GB (2 x 4GB) DDR3-2133 Memory ($79.99 @ Amazon)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive ($64.98 @ Outlet PC)
Case: Cooler Master Elite 430 ATX Mid Tower Case ($29.99 @ Newegg)
Power Supply: Corsair Builder 500W 80 PLUS Bronze Certified ATX12V Power Supply ($29.99 @ Newegg)
Optical Drive: Asus BW-12B1ST/BLK/G/AS Blu-Ray/DVD/CD Writer ($66.98 @ Newegg)
Operating System: Microsoft Windows 7 Home Premium SP1 (OEM) (64-bit) ($89.98 @ Outlet PC)
Monitor: Acer G215HVBbd 60Hz 21.5" Monitor ($89.99 @ Microcenter)
Total: $728.87
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2013-06-06 17:15 EDT-0400)
This system is a very capable one as well, it uses an APU, and has more RAM and a Blu-Ray drive. It won't be quite as good on the top end as the system above, but it would still be fine for his needs.