Oh, let him. Just make sure you do the following:
1.) Videotape him drooling all over himself when he gets the car all waxed and cleaned, etc. after he’s had it painted.
2.) Make sure you get him jabbering incoherently about how wonderful it looks.
3.) Videotape him racing the car, make sure you get a good close up of the car when it gets it’s first ding.
4.) Videotape his shattered reaction when he gets out of the car and sees how bad the damage is.
5.) Replay him the videotape of his being as giddy as a schoolgirl when he first got the thing painted. (If you’re really mean, you can videotape his reaction to this and replay it for him later.)
6.) The next time he talks about doing something stupid, remind him you’ve still got your videocamera. 