Longer rides than 15 minutes are helpful. Treats. Securing him so he can't get out and opening the window so he can put his nose out when you aren't on the freeway. Some animals are comforted by being in their crate, assuming you have crate trained him. Also, just going on long trips where he gets bored with being nervous and ends up lying down and going to sleep. Tazlina was a mess when we left California for Alaska, extremely nervous. Had calmed down a lot by the time we got to Anchorage a week later. Here is our very nervous border collie back in 2012 somewhere in Canada https://eloisetwining.smugmug.com/Travel/n-5WBmGJ/Alaska/Alaska-2012/i-SLbWFNB/A