Hey, one of mine also cracks his mouth, but that is due to an infection from tearing all the leaves off a fake plant. Never use peroxide, that will destroy good tissue as well as bad. Use a q-tip, with a solution of betadine and water, 1:10, or iodine and water, and clean out his jawline. When they mouthbreathe like that the old food gets stuck and bacteria in it. Anyway, clean out his mouth with that solution, and apply neosporin with a clean q-tip, and it should heal up.