At 16 I'm going to guess his insurance will be like $400 a month.
On a second note props to you for working your butt off to buy something you want..
BUT I would suggest you get more driving time under your belt before you get one. You don't have much driving experience and with one wheel in that back it's easy to try and over correct it if you don't know what your doing.
You're genuinely telling a 16 year old boy not to buy one? Jesus... if I had a slingshot at 16 I would need a hip replacement by 17.