Ok - so many questions. First, when does Duke usually get into the car? Is it to go places he may not like going (like, only to the vet's office?). Does he get car sick? Are there reasons he doesn't like getting in the car? Can he physically jump in the car (meaning, no physical limitations that you know of, jumping in doesn't hurt him in any way)? That's the first thing. You want to associate the car with something fun, rewire that head of his from "noooooooo, not that car, anything but the car, pleeeeeeez nooooo", to "oh, cool, the car, let's hit it!". Make plans to turn the car into training sessions, give yourself loads and loads of time, just like would when working on Sit, Stay, Down, anything else. Is Duke food motivated? I'm hoping your answer is a resounding YES! Get something really high value like chicken, hot dogs, cheese, something he LOVES, not something out of a package. You want to pay him well, since the car is obviously something he fears (for some reason). Don't be in a hurry, and don't plan on going anywhere. Put on Duke's leash, maybe get a book or a magazine (or your kindle, or your ipod) - and your full bait bag. Head to the car, open the back door, and toss in something great. Keep tossing. See if you can coerce Duke in. If not, you may have to 'help' him in, then get in with him, and just relax with him for a few min. Sit with him, pet him, talk to him, feed him some fabulous treats, explain how wonderful the car is, maybe bring his favorite toy, enjoy the back seat with him. Then get out. And do it all again. And again. And again. Session over. Now go play with him. And then tomorrow, do it again - rinse, wash, repeat. Keep it up until he (hopefully) will be able to jump in the car by himself - and still, when (hopefully) that day happens, sit with him, then get out. Finally, then
you move to the front seat. And sit for a while. Then get out. Then move the session along to starting the engine - and turning it off. Session over. Treats. Out - and a play session. Once he can jump in, he's relaxed, and you can drive, then go somewhere fun, even if it's two blocks away and you get out and go for a walk, and come home. You say he's fine once he's in, but since he obviously is an anxious dog, and has some kind of anxiety over the car, don't go too fast with him once he finally does get in. Go slow with him. I think it would be better to go slow and "get it right" than to rush, and have to backtrack, and/or start all over. Keep his rides short and sweet for a while, so he truly learns to love the car.
Sometimes (as long as it's not car sickness or hurting him to physically jump in - that's a whole different ballgame) they get weird stuff in their heads (just like we do) and we have to convince them that cars are ok. Or, sometimes altho we may not realize it, they may only associate cars with going to get shots, or go to scary places, places that smell funny, have scary sounds, etc. We need to make sure that we make car rides fun for them. He's still a very young dog and something could have happened
in the car during a fear period. Use all the patience you have, take your time, make yourself comfortable, don't rush him, and make it well worth his while (with toys, pets, love, and loads of great stuff in that bait bag!!) - my bet, he'll be just fine.
I mention the "physically" getting in part, because I have Germ Sheps - and my girl has had a back problem since I adopted her at 2 yrs old. I have an SUV now and she has a ramp to get in - BUT - at age 4 she suddenly one day refused to get into my car (a Camry sedan). HUH? She LOVED going for car rides, but always climbed onto the floor first, then the seat (she always looked funny squeezing/stepping that big body onto the floor behind the seat, but whatever). She totally had me confused. Then it dawned on me - it just hurt, she couldn't step like that anymore. She just couldn't. I got her a little stepstool, and she loved it, she got right in. Who'd have thought a GS would need a little stool to get into a car? Not a truck, or an SUV, but a car? But she did. Sometimes we need to think of all kinds of stuff - cuz sometimes they're not being stubborn, but they're trying to tell us things in the only way they know how.
I hope I've given you a few ideas, and I'm sure others will chime in also. Good luck with your boy, I know sep anxiety is difficult. Thanks for being patient with him. I'd say he's lucky to have you, but it sounds like you're lucky to have each other - and that's the way it should be.
I hate typing!