I would absolutely not let a cat loose in my car while driving. Even the sweetest natured cat can get spooked, and you DO NOT want this to happen in traffic. My Fred, now gone to the Bridge was the sweetest kitty ever, and I had been his mommy since he was a few weeks old. We were driving through Nashville during rush hour, and he got out of his carrier. He attacked me, clawed at my face, ran several laps around my neck, bit me several times all in the matter of a few seconds. I finally managed to stuff him back into his carrier, but for a split second I wondered if I might have to open the window and let him go before I had a horrible wreck involving more thatn just me. I looked like I had lost a fight with a cheese grater. I should mention that it had been a ride from NC, and he had been given meds, but it could still happen even without them.
Several years ago there was a horrible wreck here that was caused by a kitten getting under the brake pedal. 4 or 5 people and 3 kittens died whe the person driving went off a curve and hit an oak tree because she could not get the brake pedal down in time.
Unless you have a way to completely isolate from you while you are driving, having him get used to his carrier would be the safest thing, INMHO.
I am not trying to scare you or sound mean or bossy, I just want you, Jackson, and other people on the road to be safe.