" He'll eat treats and since getting desperate I've given some cold meat slices. Which he devoured. I've had to switch his food many times" .....you are making him picky!
Dogs will be like this if when they stop eating you run around and find something better... so they wait until you do, then they stop and you run around and find something else, get them a 'treat' or a taste snack.... so stop.
Find a good quality dog food and stick to it, get your dog weighed so you know what amount of food to feed and like you have meal times , no free feeding( meanig do not leave food on the floor for him to choose when/if he wants it), no treats whatsoever, no tasty extras, no added additions to the food and no bits of your food.
Lead walk the dog daily, that gets his muscles moving and his bladder/bowels, as well as socialising him, when you get back get his food, call him, make him sit and feed, if he doesn't come when called then he doesn't get the food, you put it up out of his way and try again the next meal time, same with if he takes a mouthful but then walks away uninterested, lift the food and make him wait until the next meal time.... AND NO TREATS AT ALL.... this is when you need to be strong as your dog is manipulating you.
I am suprised the vet didn't give him vitamin E especially as you say his health check is all clear....vitamin E makes them hungry, so is often given when dogs are not eating how they should... nothing stops you purchasing a 100-IU vitamin E capsules and giving him a couple a day by the end of the week he will be eating when you feed him, every time you feed him and the meal you give him.
As for 'treats' you don't give treats for nothing, you ask him to earn them by doing what you tell him to do, so new trick etc and any treats you take from their meal, you do not give 'extra' or buy the junk sold as treats or you will end up with a dog that is over weight with bad teeth and a poor digestive system as the ingredients in 95% of treats people purchase are zero use nutrienally for a dog and so very bad for dogs