Cat afraid of travelling?

Your cat obviously feel happy and secure inside your home, and prefers to be there. I would put him back inside if I was going to be gone for any length of time. All he knows is that when he wants in, he wants in. As for traveling, cats should be in a carrier, not loose in the car. Try setting his carrier out and feeding him in it for a while, so he can go in and out freely. Then you can close the door for a few minutes, and then let him out. Then pick the carrier up, with him in it, and move it to another room and let him out. When he seems calm about this, you can take him for short rides in your car, around the block for example, bring him home and let him out. You're trying to teach him that not every car ride results in a trip to the vet, and to help him feel less stressful about riding. Work your way up slowly in each step. It's not unusual for cats to meow and cry while they are riding, your goal is to just help him not be so stressed and anxious about it.