Go to a drag strip, him on his skateboard you in the Cuda, generously give him a 30s head start... hearing the roar as you fly past at 100+ mph should get his car interest going!
Its a good point you're making though and his attraction to the skateboard has an exportation date on it even if he doesn't realise. His interest in his skateboard will probably vanish at whatever age you're legally allowed to drive in your respective states. And hell, at 18 you can't rock up to your girlfriends place on a skateboard, he'll want to be rockin' up in a car and a bet he won't want it to be a Nissan Cube either, he'll want something with power and style!
For me, as a man, my car is part of who I am, I take immense pride in my Challenger and I'm sure he'll get the picture soon enough.