Many of the dogs in our agility classes were very small like Poms, Papillions, pugs, corgis, bostons, toy poodles etc. So a very small dog can do and excel at obedience. Getting the pudgy off him will make him healthier too. And no it's not hard, you progress through several levels depending on your current training level.
Even if he's not a puppy you could take a puppy beginners class. the obstacles are smaller and the focus is on learning mechanics and smaller heights etc. It's so much fun, I got hooked 4 years ago with no intention of doing so. Rally-O is also similar but all on the floor. It's really obedience and choreography like turns and really focused turns, stops etc. Google rally and see what all else there is. Freestyle, flyball, and a number of canine sports are also out there. See what you and your dog enjoy and have fun.