Just seeing this now. I'm sorry you haven't had any responses. I hope your boy is okay. Were you able to reach the vet? His emotional trauma due to the car trip, and the stay at the vet (strange smells and sounds), along with the pain of a blockage... it will likely take him some time to recover. Hopefully your vet can help you deal with the physical problem. If he has to go again, try spraying Feliway inside the car about 10 minutes before you're going to leave.
When my cat got blockages, my vet recommended giving him Dasuquin. It's intended for joint problems, but is known to help prevent UT blockages. Blacky never had another one. No way to know if it was the reason, but I've continued to give it to all my boy cats. The contents of one capsule mixed into wet food. If your kitty doesn't drink a lot of water, you can also add a little bit to make his wet food juicy. I hope you're not feeding him only dry food.
When my cat got blockages, my vet recommended giving him Dasuquin. It's intended for joint problems, but is known to help prevent UT blockages. Blacky never had another one. No way to know if it was the reason, but I've continued to give it to all my boy cats. The contents of one capsule mixed into wet food. If your kitty doesn't drink a lot of water, you can also add a little bit to make his wet food juicy. I hope you're not feeding him only dry food.