What kind of a dog is it? Some of the smooshy faced dogs (like Bostons
and bull dogs) are notorious little farters because the way their mouths are constructed, they swallow a lot of air when they eat ... and it's gotta go somewhere!
A change in food may have done it too. Does he have diarrahea too? It sounds like the food may be too rich for him, that he's not digesting it well. You're feeding him really good food, but it may be too much, too 'adult', for a young puppy. There's some really good quality puppy foods out there but you may have to go to an independent pet supply store to find them. Check on
www.dogfoodanalysis.com. No one food is right for every dog. You may have to experiment to find one that he tolerates well, just make sure that you do any change-over very gradually.
Star is right about the rice. Mine get Merrick dog food, which is
really good food, but I top it with a little cooked ground beef, rice and green beans. Without the rice, they all get a little 'soft' in the potty department. A spoonful of plain canned pumpkin (NOT the pie filling) will help firm them up too. Plain boiled chicken and rice is also good for upset tummies.
I have Bostons, and they can be the gassiest little dogs on the planet
if their food doesn't agree with them, and even sometimes if it does - they're cute little black & white fart factories! And I mean the eye-watering, sinus searing, room clearing kind of farts! A little trick - feed them some yogurt! Not the flavored kind, get the plain unflavored yogurt, and put a tablespoonful or more in their food every day. It really does work and cut down on the gas. If it doesn't, you're not giving them enough.