Why does my dog have bad gas all the time?

Poor quality food, or food that is high in fiber content is probably the most likely reason.

Some breeds are known to be more gassy than others.
I'd advise you walk dog frequently so he can eliminate and isn't holding it in. If your dog is a fast eater, that may make gassier as the dog swallows too much air by eating too quickly. (A slow feeding bowl, meal dispensing treat ball, or more frequent smaller meals may help). Hopefully, your dog is being treated with prevention for parasites, which can cause intestinal problems & flatulence.

Also, it's worth examining if the dog's input is partly responsible for the output. Many foods, including low-quality food with fillers and artificial preservatives, random table scraps, and too many snacks or foods to which they’re allergic can increase gas, so check anything that your dog is eating (food, treats) and limit or eliminate the following: low-quality proteins, meat by products, soy, corn products, cornmeal, wheat, fillers, dairy (milk, cheese, yogurt/ice cream) products, chemical preservatives, ash. Other gas producing foods are cauliflower, broccoli, any type of bean or legume, peas, beans, cabbage and bread.