I would also be careful with the really intensive things your dog is doing, like jumping into the truck. Dogs aren’t great at policing their own behavior, and if he’s already starting to slow down a little, you don’t want to add any additional concerns of trauma-related back or joint issues.
I was in a similar situation with my little dog, figured as long as he was feeling good enough to jump around he was fine, but he had some incident while being boarded and really triggered a disc issue in his lumbar spine and hasn’t been the same since 
I’m eagerly reading all these replies for suggestions for my guy, too. Definitely going to look into some of the supplements etc. suggested here, so thanks, everyone!
(My guy is on tramadol, meloxicam, and gabapentin, just as an additional data point. It gets him from “freaking out and can’t move” to “more or less getting around like normal.”)
