Sorry to read this, DM. I may have missed it but have you sought a second opinion? Your vet seems to be trying to protect his pride instead of admitting he missed something earlier.
I've found this with a certain vet chain beginning with G before now, they string out numerous possible treatments and causes to milk the cash
The minute you suggest getting a second opinion from another vet practice they miraculously diagnose correctly
and as MD says, it's a poor show from your vet when an on-line forum can diagnose a disorder without seeing the patient!
Sorry to add, though, if it is a recurring dislocation, the joint probably needs to be immobilised for a few weeks and the cat rested in a cage or similar to allow the ligaments to heal correctly, or further permanent damage may follow.
Good boy Leo for biting the vet, he must have hurt Leo to get that reaction and to pass it off as over dramatisation is appauling.