I hav emoved a couple of time with cats (myself and helped a friend). If you are going to do the 14 hour drive in one shot that is fine, but you might want to consider staying in a pet friendly hotel about half way through the journey.

If your little boy "leaves you liquid presents" on the trip to the vet, you are likely to get the same "present" here. Try using the adsorbing puppy training pads as a liner for the carrier. (That way he won't mess himself if he does leave something for you.)

Make sure the cats are wearing their collars (just in case). Is it possible for you to have them microchipped (just to be on the safe side)?

All the other advice is really good. The one thing I have learned is when you are on the highway going a constant speed cats are usually pretty good about not being to loud (otherwise turn up the radio).

Good luck!