Late 1960's Chevy (camaro, chevelle, el camino) manual transmission, that doesn't run. Get him enrolled in shop class and use the rest of the money to help him with the parts. Help him MAKE his own first car. He'll cherish it forever and a chevy small block v8 will make him feel like a man. I still have my '72 el camino and am planning a frame up resto on it in the next 10 years.
I agree with all the other posts that emphasise making sure a teenager really appreciates cars. This worked for me, 13 years ago or so. Now I can drive anything you put me in and do just about any mechanical maintenance/ work necessary for when I have an auto off of warranty. (I'm just no good with body work.. don't have the hands for it)
That's my humble opinion, but then I'm just climbing the class ladder. Some people probably expect to just have nice things on demand.