I'm not certain how you're planning on making the graphics but I'd recommend you use a digital camera to take photos of each of the body panels first.
Once you've got the diecast car in hand you should be able to measure each of the panels on it (doors, hood, trunk, etc...) and modify the pictures that you took so that they'll correspond with those dimensions. You may want to stretch and skew a few of them to get them to look right.
You can buy water slide decal paper for your printer but my experience with it was that it looks best when applied over a white background. The base color on your son's car is white already so it should be easy.
I opened up each picture that I used in a new paint program and modified them until I was satisfied. Then I combined them all onto one image file that I printed.
When I did make the printouts for that sprint car it took a number of attempts before I got it right. For some reason the dimensions from the printer weren't matching the dimensions my paint program were saved at. I ended up taking the dimensions off of my printed results and re-scaling them before I re-printed.
I tend to use a lot of aerosol paints. I'll spray them into the caps and let them sit for awhile until the paint gets to a consistency that flows well and doesn't run. Many of them are too thin to get decent coverage when they first are sprayed into the caps and will want to run. The solvents seem to dissipate quicker from some of the spray paints than regular brush-on paints so it speeds drying time a little.
