Has he been to the vet? Usually, a large, very fat belly on a young puppy means there are worms. Very, very common in puppies. If the belly is distended enough, it can cause excessive panting or trouble breathing.
The puppy should go to the vet. Lethargy, distended belly, and excess panting is worrisome.
Hi there...I completely concur with peekawho. Your puppy should be seen, not only for his health, but the health of your family as well.
Almost every puppy is born with roundworms. The symptoms you describe are consistent with a heavy parasite infestation. The scary thing about roundworms is that your puppy is shedding about 200,000 roundworm eggs into the environment every day when he defecates. Those eggs are microscopic and they are difficult to eradicate from the environment. You need to make sure that you and your kids wash your hands well after playing with the puppy or anywhere the puppy has "pooped". Roundworms can infect people as well, so it is really vital to get him to the veterinarian for an exam and dewormings as soon as possible. You can also discuss your puppies vaccination protocol at that time.
What kind of puppy is he?