Yup, probably PTSD with a fear of losing the gig.
I’ve seen some indications that PTSD in humans responds well to a few hallucinogens, ayahuasca for one and psilocybin for another.
Don’t know how well that would work for other mammals.
It would be nice to go for a walk with him, rather than being taken for a drag by him!
You might look online for their use in dogs. I know CBD is made into dog drops, so you might check out some of the online stores.
It would certainly be worth checking out if it calms him down and helps with his training.