Hi, Lee,
Your vet seems to have considered all the possibilities - tapeworm, anal fissure and anal sacs.
I think that anal sac impaction or infection is the most likely, with an anal sac infection at the top of the list. If that is the case, maybe the vet would give him a GA, and irrigate the anal sac with an antibiotic. It is impossible to do this in a conscious cat, and antibiotics by mouth or injection do not work nearly as well.
To complicate the matter, a cat's tongue is very abrasive, and constant licking his backside will in itself cause an irritation.
W.