There is some basic information on the site below about what puppies need to eat, according to how old they are:
http://www.dogbreedinfo.com/feeding.htm
(scroll down the page to see the chart.)
PLEASE do not feed your little 20-day old pup bread!
He is not old enough for solid food. He needs to be on mother's milk, or if not, then a milk supplement given by a veterinarian.
Even if he were old enough for solid food, a diet of bread is not right for a dog.
If you have been feeding bread to a puppy, it is very likely the problem has been caused by yeast in the bread dough. Puppies (even older dogs) should not be fed bread, particularly if the bread dough had a high yeast content or has not been properly or completely baked.
Why is bread such a problem ... the enzymes in the yeast convert the sugar in doughy bread to ethanol (alcohol) and carbon dioxide. These are toxic to dogs. Yeast enzymes are released ordinarily in an oven when bread is baked, but the inside of a dog's body is hot enough to release any unspent yeast enzymes too.
It is likely your dog has a huge amount of gas build-up - together with a distended abdomen. Your dog needs a vet, because leaving it in this condition may lead to multiple complications as well as the gastric tract irritation, all of which will be much harder to treat later and any one of the problems could become fatal.
Tiony
20 days old? You have a litter of pups? Sorry, need more information. You have a Vet in your area? Never heard of feeding a pup that young bread.