
When researching Mastiffs, I came across some info that said they are prone to spots on the chin if fed from plastic bowls, so if you use them in may be worth switching to ceramic or metal.
When Buster (Mastiff) eats, he has complete & tripe, he gets it all smeared in his chin, face, ears, nose, I have to give him a good wipe over after every meal to remove all the food and another good wipe over after drinknig to get rid of all the water & slobber off of his face. During the winter when he took to licking up the snow, getting his chin so wet all the time caused a spot and a sore patch to appear.
Just by keeping your dogs face clean and dry should help.
Hope he's better soon.