You might try putting him in a travel crate or carrier? You could let him get used to this in the house at first. Perhaps put a treat in it, then put his dinner in it or even his bed. If he gets used to this crate, and then the crate is in the car perhaps it won't be so scarey? Also, you can cover the crate and then he won't be able to see out and perhaps won't know he's in the car at all and will jsut sleep? Just a thought. My own dog used to sleep in his travel crate when we went to stay with friends, and it became a safe, happy environment. I think he felt safe because he was enclosed on all sides.