my son suffers from this as well, we cant use the dvd at all, we don't allow him to read, draw or use computer games. We have found that letting him sit in the front or in the middle seat helps so that he has a clear view in front of him. It also helps to make sure that he is not too hungry or full - stick to dry savoury bland snacks - e.g ready salted crisps - ham sandwiches. It depends more on the type of road rather than the length of the journey. e.g 4 hours on Motorway=fine , 30 mins on windy road=sick!
Try not to travel too early in the morning either and, not to critisise anyones driving, try to be as smooth as possible - hard acceleration and braking make it much worse.
Keep the window slightly open and the car well aired. We have not used travel sickness medication until very recently, and then waited until he has started to feel unwell before giving the minimum dose.
As for the smell - my advice is to sell the car!
Try a dose of frebreze after cleaning and keep it aired!