OK, I am both a car guy* and married to a Spendypants, so I have a little experience with this.
First, please don't write this off as keeping up with the Joneses or being led down the wrong path by a bad influence. It might be. But it also might be that he has started to go do fun things with Jack that he had never done before and realized that he really enjoys them and so wants to be able to play himself. Sort of like how I never understood why people went scuba diving when you could snorkel for free and see all the same stuff. Then I married DH, who was a Dive Master, and tried it -- and was completely, instantly hooked. Turns out it's nothing like snorkeling. I had the same reaction when DH took me to see the Mustang convertible he was eyeing -- I had no idea I liked muscle cars at all until I heard the thrum of that V8.
But the bigger picture is that even if it is just a dick-measuring contest, so what? It matters to your husband. And shaming him or lecturing him about how immature that is will get you precisely nowhere -- he is a grown-ass man, he is entitled to make dumb-ass decisions with his own money if he wants to, and becoming the Place Where Fun Goes to Die is not good for the long-term health of your marriage. You need to see his desire as meaningful to him, even if you cannot possibly think of anything stupider that he could possibly blow his money on.
Of course, that doesn't mean you need to say yes. Because, really, it is massively stupid, and it is completely selfish and disrespectful of him to change your entire financial plan for a depreciating asset that is going to cost you an arm and a leg in insurance, gas, and maintenance to boot. So your job is to figure out what it is about the car that compels him, and work with him to find a solution that will meet that legitimate need but in a way that has an acceptable impact on your budget.
So talk -- talk as if the car is a legitimate want. What is it that draws him? What feeling does it give him? Because those feelings are important and legitimate, even if the way he has chosen to get there is not.
And also make sure to talk about your other life goals and plans. How long do you both want to work? Look at the Shockingly Simple Math post and see what kind of savings rate that requires, and compare that to where you are now (assuming you transition loan payments into investments once the loan is gone).
But also look beyond the math. I think there is a tendency to focus on FIRE as a simple math problem, and so anyone who can't see the obvious impact of their choices is just stupid. But the reality is that most of us are driven by emotions. FIRE itself is an amorphous concept; what you need is a vision of what you would actually do if you didn't have to go to work every day. Do you want kids? When? If so, does one or both of you want the ability to stay home with them or work very part-time? Do you want to be able to put them through college? Do you want to take off work and travel the world? Do you have a hobby or craft that you'd love to spend all day doing, maybe turn it into a side-business? "Freedom" sounds good but doesn't really have any emotional impact to someone who is reasonably happy with his job and his life. But "OMG I would love to be able to spend all afternoon in the woodshop every day" is a powerful driver to a guy who loves woodworking.**
You need to find his "thing" -- and yours. Because the only way he is going to be able to fight off his urge for the BrightShiny right in front of him is if there is something else that he wants more in the future, and he can see exactly how much choosing this debt now is going to postpone that dream.
FWIW, I love the idea of the classic cars. Seems like Jack is big on Big and New and Powerful and Manly. Unfortunately, the only way to compete on those terms is to spend a metric shit-ton of money on something that is even Bigger/Newer/More Powerful and comes with testosterone dripping from the tailpipe. So find a way to change the playing field. But don't just tell him -- go with him and get into it yourself! Find a local classic car show and go together and look at everything that is out there. Lead him to other options that allow him to scratch the car itch and one-up Jack not by throwing more money at him, but by buying a one-of-a-kind classic model that is even cooler than Jack's.
And at the same time, set up a separate savings fund that you divert an agreed-on amount of money to each month to fund his StupidCar purchase when he finds just the right thing. It is ok to have some basic ground rules ("no debt for a depreciating asset" seems pretty reasonable to me, except maybe for your first set of needed wheels, but you may have different priorities). But you have to show him that you want to find a way for him to have the thing he wants -- that it is important to you because it is important to him. There just needs to be a compromise -- in price, in time, in type of vehicle, etc.
Really, the tl;dr is that if you act as though you think his choice is stupid, you are forcing him to dig in more to justify the rightness of his position. OTOH, if you can acknowledge that his feelings are legitimate, and show him that you want to find a way to make him happy (even if you don't quite get it yourself), he won't get his back up so much and may be more open to talking and considering a more reasonable compromise -- and then you may have to lead him to what those alternatives are so he can see for himself how cool they might be.
*For context, I currently drive a StupidCar that cost more than your husband's dream vehicle.
**Of course, the risk is that he decides cars are his "thing." ;-) Thanks to my dad's gift of the Porsche Sport Driving School many years ago, my "thing" has become driving a car very fast around a track. So for me, it is worth it to work longer to be able to afford to do that even more when I have all that free time in retirement.