Is he eating all that food with gusto?
If so, he may have EPI. He needs a blood test and you have to fast him for, I think, 12 hours prior to the blood test. They need to do a full blood panel, checking for SIBO, EPI, Vitamin B, because those all go together.
If his breath is stinky, is he eating poop? If he is very hungry -- EPI is an inability to digest the food, so it goes through without the nutrients, and the dog eats more, gets skinnier, is hungry, eats more, gets skinnier, will eat anything.
If it is EPI, there is a fix for this. You will need enzymes, your vet can tell you what, and then come back, and we can tell you where you can get them cheaper. But the enzymes need to go on the food with some warm water, and sit for 20 minutes minimum, before giving it to the dog.
Some dogs have SIBO and or a Vitamin B deficiency, and those need to be treated as well.
It can be treated and it is not necessarily terribly difficult to treat. If it is EPI. Have your vet check for it.
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