He may be dehydrated! Give him all the water he wants. Rule of thumb when your a pet owner on a trip... You know when everyone stops to use the bathroom and get a drink or something to eat? Well, your Pet needs to also.... Always think of your pet the same way you would a child, they need attention at all times.
It depends on how long the trip was. Dehydration is something to watch out for. If he starts acting funny, take him to the vet to be safe. Give him as much water as you can now. After he walks away, try to encourage him to drink more. Good luck :)
Definitely monitor the pup for the next couple of days. Severe dehydration in animals often leads to kidney failure, ensure there is no blood in his/her urine when peeing. (Just make sure it isn't coming out pink.) Also don't hesitate to give them Gatorade as mentioned above... Pedialite works as well. Finally, make sure to give them extra love for the next few days just to ensure there are no hard feelings.
Just a quick suggestion. If you stop for gas or food you can use a water bottle to pour a steady stream and allow him to drink it. This is what we do with our hunting dogs. Just make sure he doesn't try to drink from the pool of water on the ground.