Our little girl was terrified of regular toilets when she was newly potty trained so we got the Baby Bjorn Little Potty (http://www.google.com/products?q=baby+bjorn+little+potty&... ). To make matters worse, she was less than 2 1/2 and she's very small for her age so a regular toilet was even more daunting. We keep the little potty in the car and she loves using it. Even though she's fine with toilets now, we still have it in case we're going to friends for the night or are driving awhile and away from bathrooms.
Another option is a toilet ring. They do make folding ones, but I've read good and bad things about them, so we only got a regular one (http://www.google.com/products?q=baby+bjorn+toilet+traine... ). Even if you don't use the ring when you're out, it's great practice for them to learn how to use the toilet and feel confident. Once she saw her friends using a toilet ring and she learned that toilets aren't so scary, she will use regular toilets all the time we're out now. Even on vacation, she could care less about using the toilet ring, and just used the toilet (of course at home she still prefers her potty).
As for pull-ups in the car... you know how well he can hold it. If he hasn't had an accident for awhile and is really good about telling you when he needs to go, then he'll probably be fine in the car. It's amazing how quickly they learn how to hold it for a little while and we've even had to make our little girl wait a half hour or more if we were on our way home and stuck in traffic. The main thing to do is try to make him go before leaving (I know that they sometimes just won't) and remind him that you'll be in the car for awhile and will have to wait to go potty if he doesn't go now. He'll probably be fine. Worst case scenario, you'll have to wash his car seat cover. I was freaked out about the idea that she might pee in the car, but my friend said, "So what? Wash the cover," and it put it in perspective for me. It's no different from the newborn days when there was poop blow outs and milk pukes all the time - in fact, it's just a little pee. ;-) Good luck! I know how hard it is helping them with their fears at this age.