Get him a couple of those little solar powered fans that fit on the window inside the car. That should make it “cooler” 
Seriously, unless it’s in his name, let him “suffer”. He’s 18 and already got you to buy him a new car, don’t pander to his wants anymore until the car is in his name
www.carbibles.com has a calculator that will tell you exactly how much of a difference the change in size will make.
I would not use a K&N CAI system. For reasons others have mentioned.
It sounds to me like you need to take the keys and let him bum rides from his friends for a few weeks until he realizes the value of what you’ve done for him and knocks off testing your limits. And I speak as one who raised a few kids myself.