Just a very observation but when you feed a shih tzu too much wet food, his face tends to get darker and darker. Just the best will be to cut down on giving him wet food and change his diet, if possible, to just eat dry food. Just some tips from an amateur. Hope it is useful if you have a shih tzu.
By the way, if you have any photos of your little doggies, maybe you can send it to me so that I can post their pictures on my blog. Share with everyone your proud family members. Email me at firstname.lastname@example.org. Thanks and “woof woof”.