Our dog has been back and forth to France many times on the Portsmouth- Cherbourg route which is about 5 hours. He's always been absolutely fine and just sleeps. Both P&O and Brittany Ferries allow you to go down to the car deck to check on them a couple of times during the crossing (passengers aren't usually allowed down at all when the ferry sails). Other companies may have different rules but do as Chris says, take the dog for a really good walk just before you have to board and he'll be tired and will most likely sleep the whole way across (2 hours isn't really that long). Also make sure he has lots of water in the car with him and remember to leave the windows an inch or two.