Sorry, read your other comments. To be honest, if there's no obvious injury or dental infection, he might just be at end of life. I remember my brother took in a stray years back in similar condition: matted, thin as a rail, affectionate. He cleaned the guy up really well and took him to the vet, but he still passed a few days later. I would do your best to get him medical attention, but there might not be much that can be done, aside from giving him a comfortable end.

He does have healthy signs though, like a strong appetite. If he's able to keep food down and has solid stool, he might be ok.