There's also the possibility no one has mentioned yet--that the mites transmitted a disease to your snake, and it picked that moment to start showing symptoms.
Reptile Relief has more reported reactions than PAM, used correctly. I've seen bad reactions reported on various forums, though usually with lizards.
I would recommend rinsing the snake off thoroughly, and then soaking it in clean warm water, and making sure water bowls are uncontaminated.
Nervous system problems caused by a chemical are not necessarily permanent. If it was a chemical reaction, give him time, and see if he recovers on his own, don't have him put down right away. Check with a vet before you make any decisions.
You will want a professional opinion.
I would quarantine this snake, if you have other reptiles. Treat all bins with PAM--in spite of the potential bad reaction, such a thing is VERY rare, and if the problem is a disease transmitted by mites, such as IBD, killing the mites is your first priority.