My almost two year old has always hated riding in the car - she is alright for about the first 15 minutes, but even if we are going somewhere she likes, she still puts up a fuss. In fact, shes thrown such huge tantrums in the car that she's made herself throw up a few times (talk about drama!). As a mother dealing with the TERRIBLE TERRIBLE twos, there doesn't always need to be a reason for the tantrum.
As long as you know your son is dry, fed, and doesn't physically need anything, try to stay calm and ignore the tantrum as much as possible. I have a whole mess of toys that I keep in a mesh bag in the car that are toys ONLY for the car that my daughter gets to play with only in the car (books, musical toys, stuffed animals, cooking utensils, etc). I also always bring a sippy cup of water for her - sometimes, thats all it takes to head off a big meltdown. I discovered that my daughter was very sensitive to the sun on her while driving - and because of her destructiveness, we couldn't use one of those pulldown shades as she would take them apart - so now she has special sunglasses she gets to wear in the car too. I know several mothers that started bringing a portable DVD player in the car on longer trips, and while I don't use one myself, I don't see anything wrong with a kid watching an episode of sesame street instead of throwing a giant of a tantrum.
Know that you cant really prevent the tantrums, but you CAN try to turn the car ride situation into a more pleasureable experience with toys, drinks, and maybe a special song cd just for your child. Have your son help you pick out some special car toys so that you know he will enjoy the toys you give him.