HI! I work with lots of adult dogs with motion sickness problems. There has been loads of great advice given here, this is my front line approach:
He may associate the boot with feeling sick, so desensitising him to it is a must! Block of some time (like 2 weeks) where he does not have to travel in the car. Let him spend some time in the car, just a couple minuets to start with (with the engine off), then build up to feeding him in the car, giving him food stuffed Kong toys (or the pets at home equivalent). Then do the same with the engine on, gradually building up to moving the car. A crate is great if he is used to one in the house put it in the car and part cover it over.
Ask your vet for a few doses of Cerinia (a fab antisickness meds- but it is pricey..), If he has not been seen by the vet for a while they will have to do a once over before giving it to you. So the first few times he travels he does not feel sick, or you will be back at square one.
Build up the journey time and make the end really really fun!!!
An Adaptil Collar or spray (was DAP) will help the car feel more homely, 90% of dogs respond well to it. (get it off the net as it will be much cheaper than your vet!). Diazapam is a good anti-anxiety but it blocks your brain from learning new experiences, so he is not likely to remember that the last few journeys were not so bad. It could be usefull to use if you HAVE to travel him durning the time you are working on him, so as not to set you back.
Hope it helps!