If you haven't already it might be worth getting his feet checked and maybe his back. I don't know if there is one in your area but I have had success with a similar problem from a qualified equine massage therapist. Good luck
Mix of vinegar and shampoo will kill fleas on the dog, but vacuuming etc will still need doing!! Got this from google dogsdata.com. Did it yesterday on my jackapoo and seems to have worked. Now treated with advantix as ticks are a problem here. Good luck.
Not sure how far she covers your area but I can recommend Shakhura Sports Massage, slightly different to chiro but I have found that it is worth making sure I am in good order as well as the horse, She does both.:)