I doubt this is/was your cat's problem since you've had him examined, but we've had a few cats over the years with similar symptoms & discovered, each time, they'd swallowed something which was lodged in their throats (i.e., tiny sewing needle). Thankfully, their vets were able to discover the problem & remedy it. We're blessed with a great team of vets (bordering our city where we used to live) & a so-so vet (close by), so we just take our routine stuff to our neighborhood vet & our complex stuff to our A-Team across town (who also treat livestock & wild animals). And that may be a way to discern your better vets, those who "do it all" & are able to work in teams, thereby helping one another (two brains better than one).