Firstly - what a generous gift!
My only advice would be to buy something he can afford to run. I'm sure he'd love a 124 Spider Abarth but insurance and running costs for an 18yr old would be terrifying.
What's he into? If he does a lot of outdoor sports a 500X might fit his lifestyle better than a 500C, or if say he plays in a band or DJ's then a Punto or estate model might be a wiser choice to fit his gear into easier.
Failing that a nice spec 500 or Punto would be a solid choice. Cheap enough to insure and stylish enough to show off in the car park to his mates. Punto would offer more space to carry his mates about.
2015 500X Pop Star, 1.4