Both of my kids got carsick as toddlers. One seems to have outgrown it, the other still often gets carsick, as do I. My son will barf like crazy, then ask when we're having dinner. :-p
Things that help:
Not being over warm. Don't wear a coat in the car. She can use it like a blanket if he gets chilly, but being able to toss it aside is good.
Fresh air. Either a window cracked or vent-circulating air.
It also helps if others in the car do not wear perfume/cologne. To a carsick person, being stuck in an enclosed space with that makes it so much worse.
When your kid gets upset like you describe - pull over as soon as you safely can and get him out of the car. Sometimes just walking around for a few minutes will be enough to make it to the end of the trip.
Always have a bucket within reach and a container a water for rinsing it out. Having a fresh bottle of water from her to sip after upchucking is good too.
Bring a change of clothes. Also have wipes, cloths or other means of cleaning up the seat area.
Sea Bands work for a lot of people, if she will keep them on.
When she is older, he might find mint flavored gum helpful.