Superdog: The Heart of a Hero by Caralyn Buehner, illustrated by Mark Buehner

From chapters.ca:
Dexter is a little dog. He’s so little that the other dogs forget to invite him to play, and Cleevis the tomcat bullies him. But little Dexter has dreams -- big dreams. He wants to be a superhero. So Dexter starts a superhero training program. And suddenly, even Cleevis needs his help!