I worked at a bakery after high school for a little while. This was 10 years ago! But.... It was in the York, Pa area if you are close?
Anyway, most bakeries have the ability to make these "edible images" is what you are talking about. If you can find a picture that you like of a Hummer, and a Picture of your son... they could probably make one for you.
We would even sell just the image itself, for 10 bucks back then. And the people would take them home and put them on their own cakes. Or else they can do the cake as well for you.
They could drawl keys on the cake. They could do the edible image of his car or a picture of him. Just depends on what you decide you want to go with. Just to warn you, some places cannot reproduce copyrighted things. Just by law you are not allowed to. So if you have a picture of your Hummer that you owned, that might work. But if you print out something off of the Hummer website they not legally be able to reproduce it, if they were caught they could be fined. It all just depends.
I made a cake for my brother when he turned 16. He was big into football so I had this really cool picture of him in his gear drinking from a water bottle. (Almost like a sprite commercial or something! lol) And I made it into an edible image, put it on the cake, and decorated with footballs and his school colors and stuff for the writing and borders and things. He really liked it. It wasn't car related, but he wasn't into getting his license right away.
If you could get a picture of him as a toddler on his bike or old power wheels car or something, and then a "now" picture of him practicing in your car, or cut out a picture of his head and stick it in the hummer... they might be able to lay it all together and make one image from it all. That would be cute. A then and now sort of theme. The picture of him with the Hummer might be a little goofy, but thats ok b/c its the whole idea, you know?
Good luck! Hope you work something out.