I’m 6’5 as well with a 36" inseam. Having replaced my car recently I can sympathize. Unfortunately the short version of my advice is what a lot of other people are going to tell you, he’s going to have to sit in one first, and specifically sit in the drivers seat. Steering wheel, pedal and center console position are very important to what makes a car comfortable to drive.
One thing that surprises me most is how important steering wheel position is. I have driven several small cars (Corrola, Camry, Neon etc) short distances and the first thing you notice is that you frequently have to splay your right knee out at the hip (right leg is fixed at the gas pedal). In a Corrola I need to go so far out that my knee blocks my view of the radio. This makes it painful to drive for an hour or more.
In my experience tilt steering and adjustable pedals are useless to tall people, I’ve never seen a medium sized person who doesn’t like the same settings I do, usually maxed out on the floor or tilted all the way up. Telescopic steering wheels and tilt seats are useful features. In many cars, especially ones with power seats, dropping the rear end of the seat pan frees up more headroom and legroom and makes an uncomfortable leg angle more bearable by supporting your thigh.
I’ve seen some complaints here about sunroofs, I’ve only driven 2 cars with sunroofs, but in my experience it really doesn’t change the headroom at all, many car roofs have some amount of bow to them where you wouldn’t have a good forward view if your head was that close to the ceiling anyway.
At any rate, I currently drive a Pontiac G6, which feels pretty roomy but the suspension is a little stiff for roadtrips. My previous car was an Olds 88, the suspension made for a much smoother ride, but honestly I think it had less real legroom even though it was a fullsize.
Personally I would just ignore looking at compact cars, Some are surprisingly comfortable in the drivers seat, but many of those are late models with high resales. Some Nissans made in the last decade are good but that would be a little newer than you seem to intend. Trucks, minivans and SUVs are an option, but some of them are traps, late model Jeeps for example are really poorly laid out, and your son may not want a gas hog. Most fullsize cars will be acceptable, some midsize cars will work too. Check out a Taurus or Sable, any fullsize Pontiac, Oldsmobile or Buick. If your son doesn’t want to be seen driving a fullsize your options are limited, Impala, maybe an Alero, or possibly a 2 door.