Kocka used to love riding in the car as long as she was not shut in a carrier. We travelled about 400 miles and she slept a lot of the way.
I asked the vet about any kind of sedation and he said what most vets say, and that its a definate no no as cats fight the sedation and it makes them worse.
I would definately discuss the sickness with your vet if he is good.
I had a harness on Kocka and she travelled in half the carrier on my knee and she used to go for walks at service stations
She also watched out the window a lot.
When you have had Munchkin in the car was he able to see out or was the carrier covered? I ask this because if he is a travel sick case then seeing out maybe the problem. Kocka travelled in the front of the car cos I have to being a travel sick victim.
However it could be vibrations in the car.
If he is sick cos he is scared then one thing you could do is start taking him out on loads of small trips so he knowsw he is not going to the vets and that there is nothing to worry about. If this works then you will need to extewnd the length of the trips.
I was lucky cos Kocka had travelled to Devon (another 400 miles plus) on holiday with me only a month before I moved so knew she was fine but she hated travelling in a car to vets cos sadly she had many trips.