Oh poor little Murphy… This does not sound like much fun for anyone! Good on you for recognising the behaviour and wanting to get to the bottom of it.
I would suggest heading down to your local vet to have a proper work up done. It is possible Murphy could have Feline Hyperaesthesia Syndrome; however, there are many triggers for this condition, including skin disease, spinal pain, gastrointestinal diseases, muscular disease, epilepsy or behavioural conditions. Your vet will be able to assess Murph and potentially wish to run some tests to see what is causing him to attack/traumatise his tail in this way.
I hope you and your vet together can get to the bottom of what is triggering Murphy to attack his poor little tail. Let us know how you get on! Best wishes.