He's going to be too big to sit in your lap while driving so I would stop that now. I would put him in the car. Praise, possibly treat (unless he's a puker) then end the session. I would do that several times till he feels comfortable, then work up to having the car running. Eventually go around the block.
My 2 girls are truck CRAZY and my male not so much. When he was a 8wk old puppy we had to drive home about 6 hours from PA. I tell my husband it was his driving that made Apache so scared of trucks. Now when I call the dogs to go for a ride the girls come flying of course and Apache thinks about it for a while. I think he doesn't want to be left behind so he joins in all nervous and such. He knows the ride is worth the end destination, specially if it happens to be the beach.
Lakota - WGSD 1/13/10
Wolfen - WGSD 12/31/14
Bull -WGSD 2-10-10
Apache - Shiloh Shepherd 12/15/02 - 6/9/15
Kiya - Shiloh Shepherd 5/15/04 -12/19/25