The Evo is a faster car and handles better. But it costs a lot more to maintain. And insurance on it is a lot higher because so many young kids who have one get into accidents and get so many tickets their license is revoked. So I would not recommend it as a first car. Keep in mind that if he's under 18, YOU are liable for any accident he gets into. And it affects your insurance rates too.
The G6 GXP is a fast car too. Although not as fast as the Evo. Maintenance costs a lot less and insurance is not nearly as bad as on the Evo. But it still can get your son --and you-- into trouble. You need to carefully think this over.
Since you only have the two cars in mind to buy him, I'd choose the G6. But like i said, think it over carefully. If he's a minor, you are the one liable for what he does while driving it.