This is how it is with almost every friend I have, us included.
The person who carts the kids the most has to have the "kid" car. If dad takes kids to child care and off to school and back each day then he needs the car that carries the kids. If mom takes kids everywhere every day then mom has to have the vehicle that holds the kids comfortably.
It's that simple K.. If you transport the kids YOU have to have the kid friendly car. If he wants to keep your car because it's better on gas or lower payments then he has to use his for a trade in. Or you have to use yours.
It is that simple. The kids get the better car/van/truck/vehicle. Whomever drives the kids around gets to drive that.
If you guys are going to argue over this then perhaps you both need to get rid of your current vehicles and both get vehicles that are wide enough for this configuration of car seats and teenager.
I can't imagine this is comfortable for any of them. Why don't you sit in the backseat and let the teen sit up front a few times to get the feel of it. I bet both of you would have a different opinion after just one ride to the grocery store.
Mini vans are a good choice for many reasons. They are often good on gasoline, they are wide enough for several car seats across. They even sometimes have integrated car seats in the middle row. Mine has 2 of them. I can actually not have a free standing car seat in my car now. If they are forward facing they can flip the seat down and access the built in car seats.
Like this one in this picture. The seat can actually move/tilt back a bit so their head doesn't hang to the side when they doze off to sleep.
http://www.qualityvansales.com/images/seating/ICS.PDF
I know there are pro's and con's to both kinds of car seats but I love the integrated seats. I like being able to fold them up if I have grown ups going somewhere with me and not having to take the car to the car wash to vacuum and scrub out the seats under the car seats.
There are a lot of options out there that are not mini vans but truly, Suburban's, Tahoe's, other full size SUV's are the only other option for a family of your size and composite. This teen needs to have room to grow and be comfortable. If you put yourself in the back seat when riding and are completely comfortable then he should be comfortable too.