Err you didn't state acceptable budget range so I'll start with 1.5k then.

Intel Pentium G3250 - RM 235
Asus H81M-K - RM 240
Corsair 8GB DDR3 1600MHz Value RAM - RM 209
Gainward GT730  1GB GDDR5 - RM 249
WD Blue 1TB HDD - RM 225
FSP Hexa+ 400W - RM 155
Thermaltake Versa H25 - RM 139

Total: RM 1452 (Techview price inclusive of GST)

G3250 + GT730 should be able to let you run Dota 2 and CS:GO at probably medium settings at 60 FPS, lower down the settings more if you want consistent 60 FPS since you don't mind graphics settings.

I assume you already have a monitor to reuse.

TF2 as in Team Fortress 2 right? If yes, then 2k budget is able to build a rig to run it, the system requirements for TF2 is quite low. But for video rendering/editing part, quad core CPU (i5 and above) is recommended, it will still render/run on dual core (i3 or Pentium G), but it will take longer time to complete the task.

I am afraid that your budget might not be able to fit an i5, if you try to squeeze in i5, you end up with low end GPU, which is still possible to run TF2 at high FPS, but probably at low settings, if your son is ok with this then I'll go on.