If a dog suddenly starts peeing oddly and never did before, he really needs to see a vet for a potential issue like a UTI, which can be incredibly painful. Is he neutered? If so, did this start to happen after the neuter, which would indicate something going wrong there possibly?
As far as food...it sounds like you've tried all the tricks...and that he's still winning
. I don't have any helpful add'l suggestions, other than to rotate foods to keep him interested...?