why does my puppy have BAD gas?

change the food to another brand

Change his diet to a completely natural dry dog biscuit. His food is most likely the cause of this.
Unfortunately, some dogs do have terrible wind. They can’t help it. I bet if you asked your friends if you smelled when you had wind they’d agree that it smelled just as bad!

Take care!

Try changing his food. Use a premium brand of food that uses a single source of meat protein. Many foods may say Lamb or Chicken or whatever on the front of the bag – but read the ingredients and you will see terms such as meat and animal derivatives. This means that although the named meat will account for a portion of the meat content, most of the meat will be derived from other unnamed sources. The more different sources of protein you feed your dog, the greater the chance you are introducing into his diet the cause of his flatulence.
I found with my springer spaniel chicken caused him to be flatulent, but he was fine on lamb, so a little experimentation with different “flavours” may help.
One other tip is to feed him a spoonful or two of yogurt with his food. Must be live or bio yogurt. This will keep his digestive system seeded with the good bacteria he needs to digest his food properly.