baby belly! that is one of my favorite things about puppies! you should cherish it while oscar has it! he is still very young so you can expect it to be around for a bit longer. by about 4 or 5 (maybe even 6) months the pup should start looking more adult in stature -- bye bye belly!

ps -- i don't think it warrants trying to "exercise it off" -- an hour long walk for a 13 week old? whoa. this is just one of those great puppy features that, in my opinion, disappears all too soon.