My dog is suddenly afraid to get in the car. How can I help him?

Poor Hunter! It sounds like you've gone through a lot, and that fear may have manifested into new issues like you're seeing.

I'd work on getting the part of the truck you want him to ride in to be "fun" again. Generally, this is done in steps with positive reinforcement. If Hunter has a favorite toy and treat, these are also helpful. You want to make the treat something special and not often given, such as pieces of hot dog, or lunch meat, and ONLY use them when training the association that truck = fun.

You can start by just playing with him with his toy and giving treats nearby the truck for a few minutes at a time. From there, you can start trying to coax him to the truck by placing high value treats at the edge of where you want him to hop in, praising when he goes and eats them. You can then move the treats further back into the truck while asking him to hop in and get them, again, praising each time. From there you can move on to hooking him into any safety harnesses, and then taking short trips. This can be a long process, and if Hunter is scared by any step, going back to the previous and trying again from there is best.

If he still is having trouble, or seems extra fearful of the situation, speaking with your vet and a local behaviorist/trainer is a good idea. They can both rule out any health issues such as joint pain, or monitor what is going on in person and recommend a tailored training plan to help get him comfortable again.