First refer your son to this site so we can help him steer clear of "blinging" a Focus. For your price range you could get him a real exhaust, but since he wants to be "show" instead of "go", you could look in the BST section for appearance upgrades such as SVT front bumper, sideskirts, SVT wheels. Maybe even a decent set of Machs(headlights). You can get his car more low for that price too, check into the FRPP kit, or Eibach sportlines for about a 1.5" drop.
It sounds like his engine is a 2.3 Duratec which is an awesome find for that year, they take bolt-ons quite well - hopefully you can get him into performance.
Try to keep him on the OEM+ and he will appreciate it more.