This is gonna sound a bit odd, but bare with me.
You obviously want to put some money into the car. Your son obviously knows he wants a tweaked car. So, in an effort to get things done around the house (mowing the lawn, shoveling, taking the trash out, etc) you can offer to put things on the car in exchange for his work.
If he helps you reroof the house and really puts some work into it, you get suspension. If he goes out of his way to help an elderly neighbor, get a short shifter. Positive reinforcement for the kid, you get stuff done around the house, a kid that behaves, and you have a hookup up car while it's still in your hands.
PLus, when he gets the car, he knows he worked his ass off to get all the stuff on the car, so he will be much more appreciative, and he'll work harder to take care of it.