[cypher]

Did the vet give you any idea what it might be after ruling out mites?
How does he seem other than the hair loss? I can see why you're thinking it could possibly be cushings but you would be seeing additional symptoms if it was, he's not that old but some do age quicker than others so it just possible it's age related.
Without some idea from the vet what might be causing it it's hard to say what may help but you could try giving just the tiniest drop of evening primrose oil & flax seed oil or a mixed omega oil on his food as these are both very good for coat & skin condition.
Brewers yeast is supposed to be good to but I've never been able to get mine to eat any so I can't say if it helps or not from experience!