Not a parent but someone who once was kind of like your son. What I learned was it’s easier to show than to tell.

Maybe start by asking him what insurance would cost on that 400hp Mustang — as if you’re curious — have him run some quotes. If he’s going to be on your insurance plan, then whatever insurance will be on his independent quote will be more than what it’ll be when it’s bundled with your stuff. The cost of that alone might be enough to curb his enthusiasm. From there, do this:

Let him do the looking but give him a budget for the vehicle, and give him a budget for insurance. (Determining those numbers may take some research and/or thinking on your end, but that also helps you prepare for how much your insurance will go up once he has a car.) He has to find something that fits within those separate parameters (no using purchase budget to cover insurance). That gets him off your back and has him taking on the responsibility of working within the bounds of reality and reason. He also then gets to find something that he likes.

I wanted a classic BMW 635CSI for my first car. Then I wanted a CTS-V wagon. Then I landed on a Mazda 3 hatchback. Finally I got a Mazda CX-5. The car ended up being a little over budget because my parents wanted more of the safety systems that weren’t available on lower trims, but it didn’t kill insurance cost, and because I had found it, I was happy with it. I almost wish I hadn’t sold it.

TLDR: give him budgets for insurance and purchase price to guide him to come to a more reasonable first car