I am not sure how far you will be traveling, but we occasionally take a 4-5 hour drive to visit family with David. He will be 3 in April and as he has gotten older he has ended up with less blisters from the trip. I generally like to make sure part of the trip is a rest period for both of my kids for my own sanity!
The only thing I concern myself with as far as EHK goes is making sure to stop along the way to relieve some of the pressure. We will stop to eat, or shop, or play in a park. Also I make sure I have creams readily available incase he gets a terrible case of the itchys (if it is really bad a little benedryl). The only other thing I try to do is space out the times that he has to be in his car seat, so our trips are usually over a long weekend.
If you have any questions let me know, I am happy to help if I can. Good luck!