I know that some of the cats where I volunteer have reactions to plastic food dishes and get rashes on their lips or chins. Just another idea.
what do you mean when you say you've tried everything? have you changed the type of litter for starters? what do you use to wash your floors with, since that would be in contact wiht his paws...or the carpet, have you used something to clean the carpets with?.....food changes? what all have you done?