Travel is bad. Either he's excited and wired up or he's tired and cranky. Try using some of those old tricks (singalongs, eye spy, etc.) to keep him occupied and perhaps adjust seating arrangements so he and big sis are not next to each other.
Blow outs - could it be the water? Did you bring from home or fill at the campground? If at the campground, try bringing from home. Even the slightest change in mineral content can cause a reaction with little ones.
If you brought the water from home, then it is probably just nerves. Be prepared each time but know the trip will come when it stops happening.