Vit B12
It encourages appetite.
Some vets can give an injection if you think that he's not going to tolerate
tablets etc due to digestive upset. Might be worth checking out?
It possibly soounds like his sense of smell has gone if you've tried all of those.
Perhaps you need to try a really 'stinky' smelling food that he might just about
be able to smell?
What about putting a water soluble vit/mineral supplement in his water if he's drinking well?
Bit like complan - but without the milk.
There is a doggy soup called Liquivite he might lap that up? Not sure how expensive it is.
http://www.liquivite.co.uk/Hope you find the magic solution soon.